The bridge, which connected Florida International University to a student housing area over a six-lane highway in Miami, collapsed at about 1.45pm local time, crushing a number of cars below.
The operation has shifted from rescue to body recovery, with engineers fearing the support structures at either end of the bridge could also come down, Miami-Dade police spokesman Alvaro Zabaleta told reporters.
"The entire bridge is in jeopardy," he said.
The walkway, which connects Florida International University to a student housing area, went up less than a week ago but was not yet operational.
Ten people were taken to hospital, Zabaleta said, with emergency crews working all through the night at the scene.
Video footage on CNN shows the concrete structure suddenly crashing onto the road below.
At least eight cars were trapped when the 950-ton (tonne) bridge suddenly gave way on Thursday, Miami-Dade Fire Rescue Division Chief Paul Estopinan said earlier.
Police detective Juan Carlos Llera said when the bridge came down, it "sounded like an explosion. A huge bang."
"It looks like a disaster area. It looks literally like a bomb went off," Llera told AFP.
Loose cables could be at fault
The bridge was suspended from cables which had come loose and while they were being tightened, the whole thing collapsed, Florida Senator Marco Rubio wrote on Twitter.
The bridge had only been installed on Saturday, ahead of its planned opening in 2019.
More than 100 emergency services responded to the incident, who say that a search and rescue mission is ongoing.
This video, tweeted only seconds after the collapse, shows people running from all directions to help.
Sweetman Mayor Orlando Lopez described the collapse as a "national tragedy".
This soon-to-be iconic, staple part in the connectivity between the city and the university, has actually turned out to be a national tragedy Mayor Orlando Lopez
Mr Lopez described the first responders as "the true heroes".
The walkway weighed up to 950 tons and was more than 50 metres long.
In the moments after the collapse, firefighters climbed on the wreckage of the bridge searching for the injured, as paramedics treated several people on the road.
Lt. Alejandro Camacho, a spokesman for the Florida Highway Patrol, said in a telephone interview at the scene that he did know how many people had been killed, but added, “there are going to be several, based on the amount of vehicles that are underneath.”
Emergency services confirmed there are fatalities, but wouldn't say how many.
Student Aura Martinez was having lunch in a nearby restaurant with her mother when a waitress told her the bridge had collapsed. She ran outside and helped pull a woman out her car, most of which was flattened by the bridge.
“Her car, it was literally a miracle of God, her car got squished by the bridge from the back, so she was able to get out and she was on the floor and it was just very traumatic,” she told the local CBS affiliate.
Florida International University released this statement.
Florida University Statement Source: Twitter
Television imagery from the scene showed a crane being moved into position where the bridge lay in sections across Southwest Eighth Street, as emergency vehicles and police cars swarmed the area.
The $25 million bridge, which was expected to be completed in 2019, was funded through a 2013 US Department of Transportation grant, according to the Miami Herald.
Only three days ago, Florida International University tweeted this timelapse vision of the bridge being put into place.
The company which constructed the bridge, Munilla Construction Management, released this statement:
"Our family’s thoughts and prayers go out to everyone affected by this terrible tragedy.
"The new UniversityCity Bridge, which was under construction, experienced a catastrophic collapse causing injuries and loss of life.
"MCM is a family business and we are all devastated and doing everything we can to assist.
"We will conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on scene in every way."
White House spokeswoman Sarah Sanders said President Donald Trump is aware of the collapse.
“Our brave first responders are working feverishly to save lives,” she said.
Florida Governor Rick Scott's office said he will head to the scene later in the day, while the National Transportation Safety Board it will send a team to investigate.
- With Reuters
