All times are AEST. The most recent update is at the top of the page.
SUMMARY OF EVENTS:
- Three people have died and over 130 are reportedly injured after two bombs exploded almost simultaneously and about 100 meters apart, near the finish line of the Boston Marathon at 2.50pm local time.
- One of the dead is an eight-year-old boy and among the injured is a nine-year-old boy, a seven-year-old girl, and two other children aged 12 and two.
- Injuries are described as extremely serious, with some victims in a life-threatening condition, and there are several reports of loss of limbs.
- Almost 27,000 runners took part in the race, which attracts more than 500,000 spectators.
- The FBI has taken the lead in the investigation, which a White House official has called "an act of terrorism". The Department of Homeland Security, the national guard, state and local police and the ATF are also reportedly involved.
- There were unconfirmed reports police are speaking to "persons of interest." When questioned, an FBI spokesman said they would not comment. Boston Police said there is no one held in custody, despite earlier reports to the contrary.
- The city is on heightened alert, and it "will not be business as usual" tomorrow, as authorities urge residents to exercise vigilance and patience.
- Police have received multiple calls about suspicious items in the Boylston street area, but no more devices have been found, said Police Commissioner Davis.
- There are no reports of Australian victims, despite several Australians taking part in the marathon.
- US President Barack Obama did not label it a terrorist attack, however he did say "we will find out who did this and hold them accountable", and that those responsible "will feel the full weight of justice." The President also said people should not jump to conclusions about who is behind the blasts before all facts are available.
16.25pm: The Pakistani Taliban, which claimed the 2010 Times Square bomb plot, on Tuesday denied anything to do with explosions that killed three people and wounded more than 100 in Boston.
"We believe in attacking US and its allies but we are not involved in this attack," Tehreek-e-Taliban Pakistan (TTP) spokesman Ehsanullah Ehsan told AFP.
"We have no connection to this bombing but we will continue to target them wherever possible," Ehsan added.
US President Barack Obama says it is not yet clear who was behind the blasts. He said the perpetrators would pay. He did not utter the word "terror".
2.50pm: Prime Minister Julia Gillard says the safety of Australians in Boston is the main concern of the government but there is no evidence any have been injured in the "brutal and senseless" marathon bombing.
"At this stage we are not aware of any Australian victims, so we do not have any evidence of course at the moment that any Australians have been hurt in this incident," Ms Gillard said.
"Our consulate-general in New York is continuing its inquiries and the Department of Foreign Affairs will be providing updates as it can."
2.37pm: A television station in the US is reporting that police are searching an apartment in a Boston suburb, and authorities confirm the search is part of the investigation into the explosions at the Boston Marathon.
WBZ-TV reports that police are searching the apartment in Revere.
Massachusetts State Police have confirmed that a search warrant was served Monday night but provided no further details.
1.04pm: Boston Marathon competitor Phil Vaughn knew something had gone wrong when police stepped onto the course to stop runners.
“I knew that they don't stop the Boston Marathon for anything trivial.”
“I was one of the first people to be stopped, and I looked back after about 20 seconds."
"Runners were just piling up behind us; there were still a lot of runners of the course.”
He describes the next few minutes as confused and chaotic.
“We started seeing SWAT teams going by us into the finish line area… and then we started seeing spectators come running away from the finish line area towards us, and they looked like they were panicked.”
Read more of Mr Vaughan's story, as told to SBS journalist Rhiannon Elston, here.
12.01pm: Oscar Otero was near the blasts and described his experience to the Boston Globe.
“You heard boom-boom, then people screaming,” Otero said.
“There was blood all over the place. I saw a leg, people with bones sticking out of their skin,” he said.
“It's hard to describe.”
11:58am: The Boston Globe has an illustrated timeline of today's events. You can see the full timeline here.
11:47am: Police have received multiple calls about suspicious items in the Boylston street area, but no more devices have been found, said Police Commissioner Davis.
He said reports of suspicious packages were still coming in. “I am not prepared to say that we are at ease at this point of time,” he said.
11.44am: Boston student Patrick Loggins was on a train travelling under the city when two explosions above ground caused services to stop. He speaks to SBS journalist Rhiannon Elston.
11.23am: There are reports (unconfirmed by authorities) that counterterrorism officials found another five undetonated devices in the area, according to the Wall Street Journal.
11.16am: Boston Children's Hospital has reported that among the victims they are treating, are a nine-year-old boy, a seven-year-old girl, and two other children aged 12 and two.
11.05am: The Federal Aviation Administration earlier enacted a No Fly Zone in Boston airspace and temporarily stopped aircraft on the ground at Logan airport while they rearranged the runway configuration. The NFZ has now been lifted.
10.57am: A third person has died after the blast, the Police Commissioner has confirmed.
10.56am: "There is no suspect and Brigham & Women's hospital," said Police Comissioner Edward F Davis. He said police are talking to people but reiterated there is no suspect as was earlier reported.
"It is an ongoing investigation".
In response to a question, the FBI said they will not comment on whether there were "persons of interest."
10.49am: The FBI has officially taken over the investigation, it has just been announced.
At the same press conference Massachusetts Governor Deval Patrick said "it will not be business as usual" in Boston tomorrow.
Governor Patrick asked for vigilance from citizens and patience, as -- among other security measures -- there will be random checks of backpacks and parcels by police.
10.37am: The Boston Globe is reporting police are speaking to a "person of interest" at a local hospital in relation to the bombing. The individual was hurt in the blast.
10.19am: Boston Police Commissioner Edward F. Davis replying to a question of whether the explosions were a terrorist act, “We're not being definitive on this right now, but you can reach your own conclusions based on what happened.”
Davis said there was no specific intelligence on an attack and the blast occured without warning.
10.12am: SBS journalist Santilla Chingaipe talks to Indigenous Marathon Project founder Rob de Castella, who is in Boston with Darwin runner Emma Cameron, who was still competing at the time of the blasts.
10.03am: The bombs contained ball-bearings, according to CNN reports.They report the injury toll is now at least 132, including eight children. 17 are in a critical condition and another 25 are listed as serious. The source of the ball-bearing information also told CNN there have been at least 10 people who have lost limbs.
9.55am: "We are hopeful that no Australians are among those injured or dead," Foreign Minister Bob Carr told reporters at Sydney Airport earlier.
Senator Carr said Australian officials in the US were contacting hospitals, local government and marathon organisers to determine if any Australians had been caught up in the explosions.
He said concerns that the incident was a terrorist strike were legitimate.
"We know that home-grown terrorists have been a feature of life in Europe, the US and here," Senator Carr said.
"It's legitimate to be concerned about the prospect that this does represent a domestic terrorist strike."
9.35am: Many spectators abandoned bags and belongings in the panic of the blast, and bomb technicians are now apparently having to inspect them all as potential suspicious items. (Picture: Getty)
9.21am: The Boston attack is the second deadly bomb strike against a marathon, with 14 people killed in a suicide attack at a Sri Lankan event in 2008.
9.07am: An eight-year-old child is one of those killed in the blast.
Two people were confirmed dead, and the injury toll is over 100, including several life-threatening cases.
9.02am: The last mile of the marathon was dedicated to the victims and families of the Sandy Hook Elementary massacre, and several parents were in the crowd near the finish line, reports the Atlantic Wire.
8.59am: WCVB has kept track of hospital reports of blast-related admissions, and has put the injury toll at 105, including several serious or life-threatening cases.
8.43am: Reports of a suspect under watch at a hospital are incorrect. Boston Police told the New Yorks Times they are questioning multiple people but no one is in custody.
The NewYork Post earlier incorrectly reported the arrest of a 20-year-old Saudi national in connection with the blasts.
8.38am: "Our condolences go to the families of those killed and our thoughts are with those who have been injured," Prime Minister Julia Gillard said in a statement.
"It will be some time before we know the full extent of what has occurred but these explosions have cast a long shadow over one of the world's great sporting events."
8.34am: Security around the US East Coast has been stepped up in the wake of the bombing. This picture was taken at New York's Times Square, where dozens of police are on site. (Picture: Twitter / @clairerrr)
8.31am: Paris Marathon organiser Joel Laine has denounced the attack, and said that he feared the explosions would have a chilling effect on the runners scheduled to compete in the London Marathon this weekend.
"These are odious acts which one cannot quantify," said Laine, whose own marathon passed off peacefully just over a week ago with 40,000 runners taking part.
"The Boston Marathon is a convivial sporting occasion where those taking part are smiling. There will be without doubt a climate of suspicion for a good while surrounding these type of events. I am thinking notably of the London Marathon.
8.25am: AFP reports at least three dozen were wounded in the blast. Boston Police are yet to confirm.
Edward Davis, Boston Police Commissioner, earlier addressed media.
8.16am: US President Barack Obama has addressed the White House press. He said the blast is still under investigation, and while he did not label it a terrorist attack, he did say "we will find out who did this and hold them accountable", and that those responsible "will feel the full weight of justice."
"Michelle and I send our deepest thoughts and prayers to the victims and their families," President Obama he began.
"On a day like this there are no Republicans and Democrats. We are Americans, concerned for our people."
"All Americans stand with the people of Boston."
The President also said people should not jump to conclusions about who is behind the blasts before all facts are available.
8.05am: A young Indigenous runner, Emma Cameron, who was taking part in the marathon has been located and is safe, according to the founder of the Indigenous Marathon Project.
8.03am: Boston Police say the explosion at the JFK Library is likely fire related.
8.02am: President Barack Obama will deliver a statement shortly on the bomb blasts.
THE STORY SO FAR
Two explosions struck one of America's top sporting events Monday, killing at least three and wounding more than 100 as the Boston Marathon erupted in a maelstrom of blood, screams, smoke and panic.
As cities from New York to Los Angeles went on high alert, Americans with ever-vivid memories of the September 11, 2001 suicide airliner attacks automatically wondered if the country had been hit again by terrorists.
President Barack Obama went on national television to say it was not yet clear who was behind the blasts. He said the perpetrators would pay. He did not utter the word "terror."
"We still do not know who did this or why. And people shouldn't jump to conclusions before we have all the facts," Obama said. "But make no mistake, we will get to the bottom of this, and we will find out who did this, we'll find out why they did this."
A senior White House official, speaking on condition of anonymity, said later that "any event with multiple explosive devices -- as this appears to be -- is clearly an act of terror."
At the blast scene, a horrific chorus of high-pitched wailing and screaming rang out as bewildered runners and spectators fled the carnage and debris.
News reports said one of the fatalities was an eight-year-old boy and that some of the injured lost limbs. One woman told CNN the blast was the loudest sound she had ever heard, and it made the ground shake.
The thunderous blasts struck near the finish line of the marathon, long after the winners had crossed. Competitors who were still running when the blasts rocked downtown Boston were diverted elsewhere. Some 27,000 people were entered to take part in the event.
Video footage on American TV showed the moment when the first blast apparently struck: the detonation came on the left side of the course, behind spectators and a row of colorful national flags showing how runners come from around the world to take part.
Security people in yellow jackets threw their hands to their ears as the blast took place and at least one runner was thrown to the ground as white smoke billowed upward. The already waving flags whipped violently with the shockwave of the explosion.
Grisly accounts abounded. "We saw people with their legs blown off," Mark Hagopian, owner of the Charlesmark Hotel, told AFP from the basement of a restaurant where he had sought shelter.
"A person next to me had his legs blown off at the knee -- he was still alive."
"It was bad, it was fast," he said. "There was a gigantic explosion... we felt wind on our faces... Police were saying: 'Get out, get out, leave, leave there may be more bombs.'"
Boston police chief Ed Davis raised the death toll from two to three at a late evening news conference at which other officials fended off a barrage of questions about the investigation into the explosions.
FBI special agent Rick DesLauriers did say: "It is a criminal investigation that is a potential terrorist investigation."
More than 100 were injured, Massachusetts governor Deval Patrick said, without giving an exact figure. The Boston Globe said it was at least 125.
NBC News, citing officials, reported that police had found "multiple explosive devices" in Boston, raising the possibility of a coordinated attack.
The twin explosions come more than a decade after nearly 3,000 people were killed in airplane strikes on New York, Washington and Pennsylvania on September 11, 2001.
The sense of panic in the immediate aftermath of the blasts, and fear of more explosions, was so acute that Boston authorities urged people not to congregate in large crowds.
Senator Dianne Feinstein, chair of the Senate Intelligence Committee, said she believed the blasts were in fact an attack but it was unclear if the perpetrators were homegrown or foreign.
Asked if this was terrorism, she told reporters: "It looks that way."
Security was stepped up in New York and Washington -- both sites of 9/11 attacks -- as well as in Los Angeles and San Francisco.
In the Big Apple, police said they were boosting security at hotels and "other prominent locations in the city."
The blasts in Boston rattled US markets, sending the Dow and the S&P 500 down at the close.
The Boston Marathon is one of the biggest annual athletic events held in the United States. Racers must qualify to compete and there are tens of thousands of spectators.
The race attracts world-class athletes, most of whom would have likely completed the race a couple of hours before the blast went off. The video clip of the blast showed the marathon timeclock at 4:09:44.
Hours later, the flag at the majestic white dome of the US Capitol in Washington was lowered in honor of the blast victims.