FOUR people have been found dead in the rubble of a collapsed pedestrian bridge in South Florida, officials have confirmed.

The 950-tonne pedestrian bridge collapsed over a six-lane highway at Florida International University in Miami, crushing cars underneath it. It is believed that section of the bridge was installed last Sunday.

Fire Chief Dave Downey said at a news briefing on Thursday evening that four deceased people had been found at the scene by first responders.

He also says nine victims were removed “early on” and taken to hospitals. But he did not elaborate on their conditions.

He says “search and rescue mode” will continuing after nightfall.

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 said an investigation will get to the bottom of “why this happened and what happened”.

He added that if anyone had done anything wrong, “we will hold them accountable.”

The news comes as search-and-rescue crews continue to drill holes into the debris in a desperate attempt to reach any possible survivors.

As many as 10 people are feared dead, and eight vehicles are believed to be under the structure.

A fire and rescue spokesman said more than 100 firefighters, search and rescue teams and live search dogs were combing the wreckage of the bridge for “viable victims”.

The bridge collapsed about 1.30pm local time (4.30am AEDT) and at least 10 people were taken to hospital, two in critical condition.

Florida congressman Carlos Curbelo said he had been told a stress test was underway when the bridge collapsed.

“There was some testing taking place, and it is a very good question that must be answered — why more precautions weren’t taken before conducting this type of stress test if indeed that was the case,” he said.

Rescue teams would be working into the night and perhaps longer to try to find any survivors. They would not confirm how many victims there were, but Florida Senator Bill Nelson said on Fox News he had been told several people had died.

“Our teams are in search and rescue mode, they are still searching the debris,” said a Miami Dade fire department spokesman.

When asked if there were any survivors trapped beneath the debris, a fire spokesman said: “We don’t know at this point. We are still in search and rescue mode.”

Two of the eight victims taken to hospital were construction workers who had been on top of the bridge when it collapsed.

Miami Dade police department director Juan Perez said the homicide bureau and a state lawyer were “standing by” to commence an investigation as soon as all the victims were cleared from the wreckage.

“Right now we are assisting the fire department,” he said.

“The main focus here for the fire department obviously is to rescue people.”

The bridge connected Florida International University to a student housing area, where some 4000 students live.

According to CNN International, the bridge was designed to withstand a Category 5 hurricane and expected to last more than 100 years. It is also supposed to be the first bridge in the world to be constructed entirely of “self-cleaning concrete”.

TRAPPED BOY ‘BEGGING FOR HELP’
A witness told local media he ran 300 feet (90 metres) towards the bridge after hearing a “bang” and saw “cars under the bridge — you can’t even see the tyres, I mean squished”.

Restaurant owner Lianet Sado, whose business is nearby, said her husband tried to give CPR to one of two construction workers who were tied by their harnesses to the top of the bridge.

She described a devastating scene as she told how she and her staff tried to help people who were trapped in their cars.

“There was a car that was completely crushed at the front, so it was probably safe to assume unfortunately that the driver and the co-pilot were both crushed completely,” Ms Sado said.

“But then there was this young boy in the back of the car with pool of blood and just basically begging for someone to help him.

“That is basically something that I am not going to be able to get out of my head because I am a mother of a kid and I just that boy with his hand out the window, just crying for help.

“The police officers arrived and they told me that I couldn’t be under the bridge to help this person.”

Ms Sado said she moved on, feeling powerless, and tried to offer more help to other victims.

Another witness Susie Bermudez, told NBC News she was moments from the bridge when it came crashing down in front of her.

“I was three seconds away from being under the bridge, but I looked forward. All of a sudden I saw the bridge collapse, and it started from the left side of the bridge and it really shocked me,” she told NBC News.

Isabella Carrasco, who said she arrived on the scene just after the collapse, told CNN that some cars were “completely crushed”, and there was “just a lot of debris everywhere”. She said she saw one woman get out of a car that was “just nicked” and rescue personnel performing life-saving CPR on another person in the street.

Another shaken driver, Lynnell Collins, told CNN he was about to make a right turn when “the whole thing really just came down”.

“After the whole thing broke, I was freaking out. I got out of my car and me and a few other people were sprinting over there. We started helping people whose cars were at least half crushed and whoever was easily saved.” He said he saw two trucks that were “completely crushed.”

NBC reports that the bridge was built and designed by MCM in partnership with FIGG Bridge Engineers with the expectation that it could withstand hurricane-force winds, the college said.

“We are stunned by today’s tragic collapse of a pedestrian bridge,” said FIGG Engineering Group, one of the partners involved in the walkway’s construction.

“We will fully cooperate with every appropriate authority in reviewing what happened and why,” the firm said in a statement.

“In our 40-year history, nothing like this has ever happened before.”

The $US11.4 million (A$14.5 million) bridge, shared by pedestrians and cyclists, was meant to connect the school to the city of Sweetwater. It was scheduled to be opened to the public in early 2019.

It had been erected using an accelerated modular building method that enabled it to go up in the space of a day.

The university issued a response, saying it was “shocked and saddened” by the tragic event and that it was “working closely with authorities and first responders on the scene”.

A student at university tweeted: “My school finished putting up a bridge a few days ago and it literally just fell.”

“My roommate and I heard it from our rooms and ran to the balcony.”

According to the New York Post, just days earlier, the school’s president hailed the new bridge — and said that “student safety” was a priority.

“FIU is about building bridges and student safety. This project accomplishes our mission beautifully,” president Mark B. Rosenberg wrote in a since-deleted tweet.

Florida Senator Bill Nelson said it was “tragic” and “ironic” a safety bridge installed to protect students trying to cross a busy eight-lane freeway had collapsed.

According to a March 10 tweet by the university, the bridge was the “largest pedestrian bridge in US history via Self-propelled Modular Transportation (SPMT)”.

SPMT technology provides builders “with the ultimate flexibility and speed in removing and installing bridge structures”, reports the Federal Highway Administration on its website.

At the White House, Press Secretary Sarah Sanders said President Donald Trump was aware of the accident and will offer “whatever support is needed.”

On Twitter, President Donald Trump said that he’s monitoring the “heartbreaking bridge collapse” at Florida International University in the Miami area.

Trump is calling the bridge collapse “so tragic” and praising the bravery of first responders who “rushed in to save lives. Thank you for your courage.”

The president tweeted that he is praying for “all who are affected”.

NBC reports that the university is currently on spring break and classes are not running.