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MIAMI — Homicide detectives and the FBI were working Friday morning alongside engineers at the scene of the Miami bridge collapse seeking clues about what went wrong, while beginning the grueling task of freeing bodies from inside crushed cars.

At least six people were killed when the 950-ton pedestrian bridge near Florida International University's campus came down at around 1:30 p.m ET on Thursday. One of the dead was identified only as a student at FIU, and officials warned that the death toll could rise.

"We exhausted all our search and rescue capabilities," Miami-Dade County Fire Chief Dave Downey said at a 10 a.m. ET news conference Friday. "We used auditory, visual, canines and determined there are no longer any survivors. We will work to get all these victims removed."

Authorities worked overnight picking through the mess of concrete and debris, going slowly due to the fragility of the mangled structure and to preserve evidence relevant to the investigation.

"Things that are part of the evidence are going to be placed on a separate side to be transported somewhere else as the investigation continues," Det. Alvaro Zabaleta, a spokesman for the Miami-Dade Police Department, said during an earlier news conference Friday.

Zabaleta said the fire department had relinquished the lead on the investigation to the police. Personnel from the FBI, NTSB, and the Occupational Safety and Health Administration were also at the site.

The rescue effort has involved four cranes, heavy equipment, search teams and dogs.

"You have to understand that this is a very slow process," he added. "We've been working throughout the entire night."

Officials warned earlier that eight vehicles had been trapped under the bridge, but Zabaleta said "there's the sad possibility that under the concrete there may be additional vehicles."

A car is pinned under the bridge near Florida International University. GIORGIO VIERA / EPA

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"Our primary focus is to remove all the cars and the victims in a dignified manner and not compromise the investigation," Miami-Dade Deputy Mayor Maurice Kemp told reporters.

At least 10 people were taken to the hospital with injuries, Zabaleta added. None were immediately identified pending notification of next of kin.

Among those treated at the scene was Richard Humble, a sophomore at FIU who said that he was in the passenger seat of a car with his friends at the exact moment the bridge collapsed.

"We were parked at a red light and I started to hear the bridge creak," Humble said. "So I looked up and I saw the bridge falling on top of us."

He said the roof of the car caved in on his head, squishing his neck. He frantically called out to his friend in the driver's seat.

"I was screaming her name so loud cause I just wanted her to hear it and she just wouldn't respond," Humble recounted.

Humble got out of the car to call his mother. He doesn't know what happened to his friend, who is still considered missing.

Humble added that he feels "very grateful to be alive. But I just don't feel so lucky right now."

The bridge was scheduled to open early next year and designed to withstand hurricane-force winds.

Miami-Dade Mayor Carlos Gimenez on Thursday raised the possibility that a "stress test" being conducted at the time of the collapse could have played a part in the disaster, according to The Associated Press. Two workers were on the bridge when it pancaked.

Cables were attached to the walkway to take the weight off the structure, renderings show. Florida Sen. Marco Rubio tweeted Thursday that the cables had loosened and the engineering firm requested that they be tightened.

"They were being tightened when it collapsed," Rubio wrote.

The engineering company monitoring the project, BDI of Louisville, Colorado, said in a statement it was "deeply saddened" by the collapse.

Miami-based contractor MCM Construction and Tallahassee-based Figg Bridge Design said in separate statements that they will cooperate with investigators.

Kemp, the deputy mayor, said Friday that he could not confirm that a stress test was taking place at the time of the incident. Officials also couldn't say if any safety protocols had been ignored because the road beneath the bridge was allowed to remain open while workers were on the walkway Thursday.

NTSB Chairman Robert Sumwalt said Friday morning that its investigators had yet to access the site because debris still had to be removed. He said the agency has had experience with other major bridge collapses, including the buckling of the I-35W Mississippi River bridge in 2007 that left 13 dead and led to a $52.4 million lawsuit.

The Miami pedestrian bridge over the multi-lane Southwest Eighth Street connected the FIU campus to the town of Sweetwater, where the university estimates some 4,000 students live.

The university had celebrated the unveiling of the walkway five days prior to the collapse.

Gabe Gutierrez reported from Miami, Erik Ortiz from New York, and Alexander Smith from London.