UPDATE: As of Friday morning, six people are confirmed dead.

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

The massive span — in a sudden, catastrophic failure — crashed down across eight lanes of heavily traveled Tamiami Trail, flattening eight cars. The death toll remained uncertain as rescue crews continued to work into the night to reach vehicles but late Thursday Miami-Dade fire chief Dave Downey confirmed at least four people had been killed, including a student from FIU, police sources said.

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Nine people had been pulled from the rubble by evening and rushed to Kendall Regional Medical Center’s trauma unit, including two who required immediate surgery. The others sustained injuries ranging from scrapes and bruises to broken bones, which were not considered life threatening. On campus, some families waited for word on missing loved ones.

Even before the dust from the disaster settled, motorists scrambled from their cars to help. At least one woman, Katrina Collazo, was pulled from her half-crushed car, miraculously unscathed.

“Thank God ... my daughter is alive,” said her mother, Ada Collazo, in Spanish, after rushing to the scene, fearful that another family member also might have been riding in the back. “I thought my granddaughter was in the car, but she wasn’t. She’s in school.”

Collazo said her daughter, who had been on campus for a nursing meeting, stopped at a red light when she said she heard what sounded like small rocks falling on her car. As she turned around, the span mashed everything behind the driver seat. The car next to her was not as lucky. That vehicle was flattened like an aluminum can.

It was not immediately clear what caused the collapse of a $14.2 million structure FIU had touted as an innovative “instant” bridge because of construction techniques intended to speed up the work and minimize disruption to commuter traffic. The bridge’s main 175-foot span, assembled on the side of the road, was raised into place across Tamiami Trail on Saturday in less than six hours. But the project was far from complete and not expected to open to student foot traffic until 2019.

Several witnesses reported that two workers were on the bridge when it collapsed shortly before 2 p.m. Early in the day, Miami-Dade Mayor Carlos Gimenez said the bridge had undergone a “stress test” but it was unclear what, if any, role that might have played in the failure. FIU President Mark Rosenberg confirmed there was testing on the bridge sometime before the collapse, but said the testing was proper.

“I have not spoken directly to Munilla Construction, but I am satisfied that the testing that was occurring was consistent with best practices," Rosenberg said shortly after 8 p.m. “I’m not an engineer, so I'm not privy to those details. I know that tests occurred today. And I know, I believe, that they did not prove to lead anyone to the conclusion that we would have this kind of a result. But I do not know that as a fact.”

Late Thursday, came the first definitive word from a government official about what was being done on the bridge when it fell. U.S. Sen. Marco Rubio, R-FL, said in a Twitter post that: “The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today.”

Rubio, a Miami Republican, did not elaborate on the information. But he was in a position to know inside details about the catastrophe.

He traveled to the FIU campus Thursday on same plane with Rosenberg. He said he spoke to MCM partner Pedro Munilla amid the rubble and he attended private briefings at FIU.

The late post came soon after Gov. Rick Scott and Rubio both promised swift investigations.

“We will hold anybody accountable if anybody has done anything wrong,” Scott said.

Rubio also vowed an “exhaustive” review with scrutiny of “science and engineering” that went into the project. “The families and the survivors deserve to know what went wrong,” he said.

The National Transportation Safety Board announced that it was dispatching a team of 15 to examine the collapse and investigators expected to be on site by late Thursday.

Authorities stressed that it could take days or more to determine what went wrong. But one agency, the Florida Department of Transportation, quickly distanced itself, issuing a fact sheet saying it had a limited role in the project and emphasizing FIU’s responsibility for testing and safely completing the bridge.

Designed as a cable-supported bridge, the 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 Sunshine 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 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.”

Mayor Gimenez, in Hong Kong on a trade mission, said in a telephone interview that he instructed county rescue workers to identify license plates from the trapped cars as quickly as possible so that families could be notified.

Rescue crews with specially trained dogs and listening devices were poring over the wreckage, hoping to find survivors. About two hours after the collapse, crews also brought in heavy equipment to probe under sections of the shattered span.

“I don’t know what’s under the bridge, under the rubble,” said Lt. Alex Camacho of the Florida Highway Patrol. “It’s impossible to see.”

The injured were transported to the trauma center at Kendall Regional. Mark McKenney, chief of the trauma department, said the 10 injured patients ranged in age from 20 to 50 years old, including one man whose heart had stopped beating when he arrived.

Doctors revived the man, who was not identified, and wheeled him into the operating room. He is listed in critical condition with head and chest injuries.

A second patient arrived comatose with severe injuries that required orthopedic and neurosurgery, the hospital spokesperson said. The remaining eight patients are in stable condition with injuries ranging from scrapes and bruises to broken bones.

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 opening nearby on Thursday.

President Donald Trump also posted a tweet, “Continuing to monitor the heartbreaking bridge collapse at FIU — so tragic.”

Students and faculty have long clamored for a bridge at the 109th Avenue crossing, where students on foot have to scurry across heavily traveled Tamiami Trail, which divides 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.

The pedestrian walkway was installed in a single morning at Southwest 109th Avenue on Saturday, intended eventually to link FIU’s Modesto A. Maidique Campus to the small suburban city of Sweetwater, where the university estimates 4,000 of its students live.

FIU, which has an “accelerated bridge construction” program in its engineering school, promoted the project’s innovative approach. 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. 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.

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

Miami Herald Staff Writers Howard Cohen, Alex Harris, David J. Neal, Charles Rabin, David Smiley, Jennifer Staletovich, Carli Teproff and Martin Vassolo contributed to this report.