There has been a breakthrough in the United States in the attempt to find those responsible for this week's Boston Marathon blasts.
The FBI has released photos and video of two men they consider to be linked to the bombings.
Santilla Chingaipe has the details.
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US Investigators have released pictures of two suspects in the Boston Marathon bombing.
The FBI says it has no details of the identities or origin of the two men.
Both appear to be young men, one dressed in a white baseball cap and one in a black cap.
FBI special agent Richard DesLauriers says they are seeking the public's help in finding the men photographed on the crowded footpath before Monday's bombs exploded near the finish line.
"We are enlisting the public's help to identify the two suspects. After a very detailed analysis of photo, video and other evidence, we are releasing photos of these two suspects. They are identified as suspect one and suspect two. They appear to be associated. Suspect one is wearing a dark hat, suspect two is wearing a white hat. Suspect two set down a backpack at the site of the second explosion just in front of the Forum restaurant."
Mr DesLauriers says he hopes someone will be able to come forward with information about the men.
"Somebody out there knows these individuals as friends, neighbours, co-workers or family members of the suspects. Though it may be difficult, the nation is counting on those with information to come forward and provide it to us."
He's also warned people against taking the law into their own hands.
"We consider them to be armed and extremely dangerous. No one should approach them. No one should attempt to apprehend them except law enforcement. Let me reiterate that caution: do not take any action on your own"
Monday's twin bombings near the finish line of the Boston Marathon left three people dead and injured 176 others.
The bombs sprayed nails, ball bearings and other metal fragments into the huge crowd, many of whom suffered major injuries.
US President Barack Obama has paid tribute to the victims of the blasts at a memorial service in Boston.
He also vowed that the perpetrators would be held accountable.
Speaking at the interfaith service at the Cathedral of the Holy Cross, President Obama said the nation was united in support of the victims.
"Our prayers are with the injured, so many wounded, some gravely. From their beds, some are surely watching us gather here today. And if you are, know this: As you begin this long journey of recovery, your city is with you. Your commonwealth is with you. Your country is with you. We will all be with you as you learn to stand and walk and, yes, run again. Of that, I have no doubt you will run again. (Applause) You will run again because that's what the people of Boston are made of. Your resolve is the greatest rebuke to whoever committed this heinous act."
President Obama told the congregation Americans would not be intimidated by the twin blasts.
"If they sought to intimidate us, to terrorize us, to shake us from those values that Deval* described, the values that make us who we are as Americans, well, it should be pretty clear by now that they picked the wrong city to do it. (Applause). Not here in Boston. (Applause). Not here in Boston. (Applause)."
Earlier in the week, US media reported that police were talking to a 20-year-old Saudi man who suffered a leg injury at the scene.
The man was initially under armed guard in hospital, but the Saudi embassy in Washington says he was a witness and is not accused of any involvement.
A Moroccan-American teenager has also been cleared of any connection to the blasts after images of him circulated online and on the front page of a U-S news tabloid.
The Murdoch-owned New York Post identified Salah Barhoun on its front page as a suspect investigators wanted to locate.
The 17-year old runner says he was forced to visit a local police station in order to clear his name.
"I just don't want to look at people, because when they look at me, they're going to be like 'you just did this. How could you do that? Why would you even do that? You've got so many people .. you killed eight year old you killed. You got people injured? How are their family going to feel so horrible after what you did?' But if you look at it, it wasn't me".
Meanwhile, this weekend's London Marathon will take place amid a climate of heightened security and increased vigilance following Boston Marathon blasts.
Officials say extra police will patrol the course on Sunday, which snakes alongside the River Thames and passes iconic landmarks including the Tower Bridge, the Houses of Parliament and Queen Elizabeth's Buckingham Palace residence.
