Update - 10.29am: At least six people have died after a pedestrian bridge collapsed on to a busy road in Miami.
Authorities said vehicles were crushed beneath massive slabs of concrete and steel in the incident on Thursday in which at least 10 people taken to hospital.
Search and rescue crews worked through the night, using dogs, search cameras and sensitive listening devices in a frantic bid to find survivors among the debris.
8.57am: Construction method of collapsed Florida bridge 'supposed to reduce risks to workers and pedestrians'
The "accelerated bridge construction" method was supposed to reduce risks to workers and pedestrians and minimise traffic disruption, the Florida International University said.
The $14.2m bridge crossed over a busy seven-lane road that divided the university campus from the city of Sweetwater.
The 950-ton span was installed on Saturday to great fanfare.
The main part of the 174-foot span was assembled by the side of the road and had to be moved into place.
At least four people have been killed after the pedestrian bridge collapsed on to a busy road below in Miami.
Robert Bea, a professor of engineering and construction management at the University of California, Berkeley, said it was too early to know exactly what happened, but the decision to use what the bridge builders called an "innovative installation" was risky, especially because the bridge spanned a heavily travelled thoroughfare.
"Innovations take a design firm into an area where they don't have applicable experience, and then we have another unexpected failure on our hands," Mr Bea said after reviewing the bridge's design and photos of the collapse.
The project was a collaboration between MCM Construction, a Miami-based contractor, and Figg Bridge Design, based in Tallahassee. Figg is responsible for the famous Sunshine Skyway Bridge across Tampa Bay.
Figg issued a statement on Thursday saying the company was "stunned" by the collapse and promising to cooperate with investigations.
"In our 40-year history, nothing like this has ever happened before," the company's statement said. "Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved."
MCM Construction Management, which is building the bridge, posted a message to the company's Facebook page promising "a full investigation to determine exactly what went wrong".
6.26am: Rescue efforts continue as four die in Florida bridge collapseFour people have been found dead in the rubble of a collapsed South Florida pedestrian bridge in south Florida where the frantic search for any survivors continued past nightfall, officials said.
Fire Chief Dave Downey said at an evening news briefing that four deceased people were found amid the chaotic scene of concrete rubble and crumpled vehicles.
He said nine victims were removed "early on" and taken to hospitals but did not elaborate on their conditions. He said the "search and rescue mode" - deploying trained canines, search cameras and sensitive listening devices - was continuing into the night.
Governor Rick Scott also spoke to reporters, saying "everybody is working hard to make sure we rescue anyone who can be rescued".
The Florida governor added that an exhaustive investigation now beginning will get to the bottom of "why this happened and what happened" and vowed that anyone who did anything wrong would be held accountable.
Miami-Dade police director Juan Perez praised the actions of first responders during a news conference but acknowledged the likelihood of finding more victims under the rubble was growing more difficult with the passage of time.
"We know that there's going to be a negative outcome at the end of the day," Mr Perez said.
The bridge was under construction and collapsed on to a busy Miami highway on Thursday, crushing at least eight vehicles under massive slabs of concrete and steel. One side of the bridge led to Florida International University (FIU), a school west of the city's downtown.
Search-and-rescue crews drilled holes into the debris and used their highly trained dogs to look for survivors. They had to work carefully because part of the structure was still unsafe. At least 10 people were taken to hospitals. The number of fatalities was not immediately known.
This timelapse video shows the installation of the pedestrian bridge at Florida International University in Miami. The bridge collapsed a few days later. https://t.co/e1naASZsy9 pic.twitter.com/eco0fzYEbG— CNN (@CNN) March 15, 2018
The 950-ton bridge had been assembled by the side of the highway and moved into place on Saturday to great fanfare. The span stretched almost 200ft to connect Florida International University with the city of Sweetwater. It was expected to open to foot traffic next year.
"This bridge was about goodness, not sadness," said FIU president Mark Rosenberg. "Now we're feeling immense sadness, uncontrollable sadness. And our hearts go out to all those affected, their friends and their families. We're committed to assist in all efforts necessary, and our hope is that this sadness can galvanise the entire community to stay the course, a course of goodness, of hope, of opportunity."
Jacob Miller, a senior at FIU, was visiting a friend in a dorm when he heard sirens and horns honking. He went to a balcony and could see rubble coming down.
VIDEO: Florida International University bridge collapses. View from student dorms. pic.twitter.com/4wuD89IPhj— Ana Demendoza (@AnaDemendoza_) March 15, 2018
"I saw there were multiple cars crushed under the bridge. It was just terrible. I saw some people stopping their cars, trying to get out, trying to assess the situation to see if there is anything they could do to help," he said.
National Transportation Safety Board chairman Robert Sumwalt III said a team of specialists was heading to Miami on Thursday night with plans to begin its investigation this morning.
Miami-Dade Police director Juan Perez said his department's homicide team would take over the investigation after rescue efforts are complete.
Munilla Construction Management (MCM), the Miami-based construction management firm that won the bridge contract, had a news release on its website touting the project with FIGG Bridge Engineers, a Tallahassee firm. It said initial reports that it had taken its website down were incorrect and that the site had become temporarily unavailable due to increased web traffic. "It has since been restored," the company said in a statement.
MCM said on Twitter that it was "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."
FIGG said in a statement, "In our 40-year history, nothing like this has ever happened before."
Florida International University is the second largest university in the state, with 55,000 students, most of whom live off-campus. The bridge was supposed to be a safe way to cross a busy highway.
A university student was killed in August while crossing the road that the bridge was supposed to span.