The Miami-Dade County fire chief said four people have been found dead in the rubble of a collapsed pedestrian bridge in south Florida.

Fire Chief Dave Downey said at a news briefing Thursday evening that four deceased people had been found at the scene by first responders. He also said nine victims were removed "early on" and taken to hospitals. But he did not elaborate on their conditions.

He said "search and rescue mode" is continuing after nightfall.

Gov. 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 investigation will get to the bottom of "why this happened and what happened." He adds that if anyone had done anything wrong, "we will hold them accountable."

Live press conference of <a href="https://twitter.com/hashtag/FIUBridgeCollapse?src=hash&ref_src=twsrc%5Etfw">#FIUBridgeCollapse</a> <a href="https://t.co/Z8ZcpYyge7">https://t.co/Z8ZcpYyge7</a> —@MiamiDadeFire

The 860-tonne, 53-metre span was assembled by the side of the highway and moved into place Saturday to great fanfare. The $14.2 US million bridge connected Florida International University and the city of Sweetwater. It was expected to open to foot traffic next year.

"We are shocked and saddened about the tragic events unfolding at the FIU-Sweetwater pedestrian bridge. At this time we are still involved in rescue efforts and gathering information," the school said in a statement.

The National Transportation Safety Board sent investigators to the scene. At a news conference late in the day, chairman Robert Sumwalt said the board had assembled a team of 15 people and would begin an investigation immediately.

The investigation will include specialists in the area of civil engineering and material science and will look at whether other similarly structured bridges may be unsafe.

Miami-Dade Fire Rescue Department personel and other rescue units work at the scene. (Joe Raedle/Getty Images)

"If we find that there are deficiencies with this kind of bridge," said Sumwalt, "we will issue urgent recommendations."

"We have a national tragedy on our hands," Sweetwater Mayor Orlando Lopez said.

The "accelerated bridge construction" method was supposed to reduce risks to workers and pedestrians and minimize traffic disruption, the university said.

"FIU is about building bridges and student safety. This project accomplishes our mission beautifully," FIU president Mark B. Rosenberg said in the statement Saturday.

'Bridge was done too quickly'

Cristina Rodriguez, a 23-year-old junior who was on spring break with other students, said she was not surprised when she heard the bridge collapsed.

"I just felt the bridge was done too quickly to believe the bridge was stable and sound to support everything that was on there," said Rodriguez, who was not on campus Thursday but drives through the intersection almost daily.

MCM, the Miami-based construction management firm who won the bridge contract, took its website down on Thursday. But an archived version of the website featured a news release touting the project.

THE FIU BRIDGE COLLAPSED OH MY GOD <a href="https://t.co/JO7jfx5AoN">pic.twitter.com/JO7jfx5AoN</a> —@GabrielaRose12

"This our first Design-Build with FIGG Bridge Engineers, a nationally acclaimed, award-winning firm based out of Tallahassee. FIGG has designed iconic bridges all over the country, including Boston's famous Leonard P. Zakim Bridge and Florida's Sunshine Skyway Bridge."

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 co-operate with investigators on scene in every way."

FIGG said in a statement it was "stunned by today's tragic collapse."

"In our 40-year history, nothing like this has ever happened before. Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved."

First-of-its-kind pedestrian bridge “swings” into place. “FIU is about building bridges and student safety. This project accomplishes our mission beautifully,” -President Mark B. Rosenberg. <a href="https://t.co/x8gPM9A4DG">https://t.co/x8gPM9A4DG</a> <a href="https://twitter.com/hashtag/worldsahead?src=hash&ref_src=twsrc%5Etfw">#worldsahead</a> <a href="https://t.co/mPEMeh2zmw">pic.twitter.com/mPEMeh2zmw</a> —@FIU

White House press secretary Sarah Huckabee Sanders said the president was monitoring the situation and would offer whatever support was needed.

Rep. Mario Diaz-Balart, who spoke at a ceremony celebrating the bridge's construction over the weekend, told CBS there were going to be a lot of questions that have to be answered about what happened.

"Right now the most important thing is going to be to save people who are hopefully still alive," he said.

Emergency personnel work at the scene of a collapsed pedestrian bridge at Florida International University on Thursday. (WTVJ NBC6 via AP)

Florida International University is the second-largest university in the state, with 55,000 students. Most of its students live off-campus and were on spring break when the collapse happened. The bridge was supposed to be a safe way to cross a busy street, with a plaza-like public space with seating where people could gather.

In August 2017, a university student was killed crossing the road that the bridge was supposed to span.

Florida International University is also home to the National Hurricane Center.