A pedestrian bridge under construction collapsed Thursday, just days after crews had dropped an elevated 950-ton span in place in a project that was intended to give Florida International University students a safe route across the busy roadway.

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MIAMI — A pedestrian bridge that was under construction collapsed onto a busy Miami highway Thursday afternoon, crushing vehicles beneath massive slabs of concrete and steel and killing at least four people, authorities said.

Search-and-rescue crews worked into the night, using dogs, search cameras and sensitive listening devices in a frantic search to find survivors among the debris.

"Everybody is working hard to make sure we rescue anyone who can be rescued," Florida Gov. Rick Scott said.

But Miami-Dade police director Juan Perez said hopes were dwindling as the hours passed.

"We know that there's going to be a negative outcome at the end of the day," Perez said.

Four people were found dead amid the chaotic scene, and nine victims were taken "early on" to hospitals, Fire Chief Dave Downey said without elaborating on their conditions.

The partially built 950-ton (860-metric ton) bridge had been assembled by the side of the highway and moved into place Saturday to great fanfare. The span stretched almost 200 feet (60 meters) 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."

Scott said an exhaustive investigation will uncover "why this happened and what happened," and he vowed to hold accountable those responsible.

National Transportation Safety Board chairman Robert Sumwalt III said a team of specialists was heading to Miami on Thursday night to investigate.

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.

"I saw there were multiple cars crushed under the bridge. It was just terrible," he said.

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

Renderings of the finished bridge showed a tall, off-center tower with cables attached to the walkway to support it. When the bridge collapsed, the main tower had not yet been installed, and it was unclear what the builders were using as temporary supports.

Robert Bea, a professor of engineering and construction management at the University of California, Berkeley, said it's 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 traveled 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," Bea said after reviewing the bridge's design and photos of the collapse.

Sweetwater police Detective Juan Llera was in a nearby meeting when the bridge collapsed.

"I heard a 'boom' like a bomb had exploded," he said. "At first I thought it was a terrorist attack."

He said he saw three injured construction workers. One had a head injury and was passing in and out of consciousness, another one had a leg injury leg and the third was lying on the street unconscious. Llera started performing CPR on him.

"We were able to keep him alive to send him to the hospital," Llera said.

Kendall Regional Medical Center received 10 injured people. Of those, two were in "extremely critical" condition and eight were stable with injuries such as broken bones, bruises and abrasions, said Dr. Mark McKenney, the hospital's director of general surgery.

Of the two more serious cases, one arrived in cardiac arrest but was revived. The other had a serious brain injury, McKenney said.

The main companies behind the $14.2 million construction project have faced questions about their past work, and one was fined in 2012 when a 90-ton (80-metric ton) section of a bridge collapsed in Virginia.

Munilla Construction Management, or 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. After the collapse, it said the site had become temporarily unavailable due to increased web traffic. "It has since been restored."

MCM said on Twitter that it 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."

FIGG said in a statement, "In our 40-year history, nothing like this has ever happened before."

But FIGG was fined in 2012 after a 90-ton (80-metric ton) section of a bridge it was building in Virginia crashed onto railroad tracks below, causing minor injuries to several workers. The citation from the Virginia Department of Labor and Industry said FIGG did not properly inspect a girder and had not obtained written consent from its manufacturer before modifying it, according to a story in The Virginian-Pilot.

Court documents show that MCM was accused of substandard work in a lawsuit filed earlier this month. The suit said a worker at Fort Lauderdale International Airport, where the company is working on an expansion, was injured when a makeshift MCM-built bridge collapsed under his weight.

The suit accused the company of employing "incompetent, inexperienced, unskilled or careless employees" at the job site.

A review of Occupational Safety Health Administration records shows that MCM has been fined for 11 safety violations in the past five years. The fines totaling more than $50,000 arose from complaints about unsafe trenches, cement dust and other problems at its Florida work sites.

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.

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Associated Press writers Jason Dearen in Gainesville, Kelli Kennedy in Fort Lauderdale, Curt Anderson in Miami and Tamara Lush in St. Petersburg contributed to this report.

The FIU bridge collapsing... I’m still in shock pic.twitter.com/ZNqO2z5ch6

— Megan (@meganmfernandez) March 15, 2018

The bridge crashed across six lanes of heavily traveled Tamiami Trail, crushing a still undetermined number of cars and killing a still unclear number of people. Police on the scene said at least six people could be dead.

The Florida Highway Patrol reported five or six cars were trapped under the bridge. Miami-Dade County police said at least eight cars had been crushed under the walkway, which was not yet open to student traffic.

At least eight people had been transported to the trauma center at Kendall Regional Medical Center, according to a source close to the hospital. The condition of the patients is not yet known.

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Miami-Dade County Police Chief Juan Perez said he believed there were multiple people trapped. He wouldn’t venture to guess at the number because first responders were having trouble getting to the vehicles. South Florida’s WSVN reported that television news helicopters were ordered to back off so rescuers could listen for sounds from survivors.

The collapse was clearly a major failure of a project not expected to be completed until early 2019. There was no immediate explanation for what might have triggered the collapse, which occurred shortly before 2 p.m.

MCM Construction Management, which is building the bridge, posted a message to the company’s Facebook page after the collapse promising “a full investigation to determine exactly what went wrong.”

BDI, the engineering firm responsible for monitoring the structure, said the company would release a statement regarding the bridge collapse later Thursday.

FIU spokesperson Maydel Santana-Bravo issued a statement, even as rescue crews were still working the scene.

“We are shocked and saddened about the tragic events unfolding at the FIU-Sweetwater Bridge,” she said. “At this time, we are still involved in rescue efforts and gathering information. We are working closely with authorities and first responders on the scene. We will share updates as we have them.”

FIU students are on spring break this week, but traffic was expected to be heavy with the Miami-Dade County Youth Fair nearby scheduled to open at 3 p.m. today.

Florida Gov. Rick Scott issued a statement that he had spoken with Miami-Dade County Police Chief Juan Perez, whose agency dispatched homicide detectives to the scene, which occurs during all mass casualty events.

Scott’s office said the governor will visit FIU today to be briefed by police and university officials on the collapse.

White House Press Sec. Sarah Sanders said President Donald Trump is “aware” of the deadly pedestrian bridge collapse and will “offer whatever support is needed” to local officials.

Touted as an “instant bridge,” the 950-ton pedestrian walkway was installed in a single morning at Southwest 109th Avenue on Saturday, intended eventually to link Florida International University’s Modesto A. Maidique Campus directly to the small suburban city of Sweetwater, where the university estimates 4,000 of its students live.

Before Saturday’s installation, FIU said the pedestrian bridge used an innovative approach under which a 175-foot section of the overall 320-foot long bridge was fabricated by the side of the Trail while support columns were erected in place.

Over a few hours on Saturday morning, the 950-ton span was lifted off the ground by a mechanical transporter, swung into position across the Trail, then lowered into place over the support columns.

That reduced to a minimum the time the trail had to be closed to traffic, and minimized risks to workers and people in the vicinity, FIU said.

The university’s engineering school has an “accelerated bridge” program that works on techniques to speed up bridge construction. The program consulted with builders, designers and engineers early in the process, but was not closely involved in the pedestrian bridge program, a spokeswoman said last week.

There were as yet no stairs or ramps to the bridge, which was not open. Opening was scheduled for early next year.

Designed as a cable-supported bridge, the $14.2 million bridge project was a collaboration between MCM Construction, a prominent Miami-based contractor, and Figg Bridge Design, based in Tallahassee. Figg is responsible for the iconic Skyway bridge across Tampa Bay.

Figg issued a statement Thursday saying the company was “stunned” by the collapse and promising to cooperate with every authority investigating the collapse.

“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 and Figg are part of a consortium that has mounted a legal challenge of the Florida Department of Transportation's award of the Interstate 395 reconstruction project to a rival team, holding up the plan for months.

A citizen panel judging the 395 bids on aesthetic grounds overwhelmingly preferred Figg’s design for a signature bridge over Biscayne Boulevard, a key element in the project. But MCM’s team alleged that FDOT changed the scoring method to favor the other team's submission. Figg’s design for a bridge suspended from cables features two support towers that resemble dancers.

Students and faculty have long been clamoring for a bridge at the 109th Street crossing, where students on foot have to get across seven lanes of jam-packed traffic that divide the campus from Sweetwater. Though FIU provides shuttles, many students prefer to walk. In August, FIU undergraduate Alexis Dale was hit and killed by a motorist while crossing the intersection.

This is a breaking news story and will be updated as more is learned.

Miami Herald Staff Writers Howard Cohen, Douglas Hanks, Alex Harris, David J. Neal, Charles Rabin, Jennifer Staletovich, Martin Vassolo contributed to this report.