Miami (AFP) - The chief engineer of a Florida bridge project warned authorities of cracking in the structure days before it collapsed, killing at least six people, the southern US state's department of transportation said Friday.

Chief engineer Denney Pate left a voicemail on a Florida Department of Transportation employee's landline on March 13, two days before the pedestrian walkway came crashing down on the major road beneath, the department said in a statement.

The voicemail -- which was not heard until Friday because the employee it was left for was out of the office -- mentioned a problem, but did not warn that structural failure was imminent.

There was "some cracking that's been observed on the north end of the span," Pate said, according to a transcript of the call.

"Obviously some repairs or whatever will have to be done but from a safety perspective we don't see that there's any issue there," Pate said.

At least eight cars were trapped when the 950-ton (tonne) bridge suddenly gave way on Thursday, Miami-Dade Fire Rescue Division Chief Paul Estopinan said.

The walkway, which connected Florida International University to a student housing area, had been raised less than a week ago but was not expected to be operational until 2019.

- Death toll could rise -

Miami-Dade county police spokesman Alvaro Zabaleta told reporters that on Friday rescue operations shifted to one of body recovery, with engineers fearing the support structures at either end of the bridge could also come down.

"The entire bridge is in jeopardy," Zabaleta said.

Juan Perez, director of the Miami-Dade police department later told reporters that a team of government prosecutors was on the scene as part of the investigation, but stressed it was too soon to say whether criminal charges would be brought.

"It is important that we understand, this is a homicide investigation. That's all it is," he said. "That means that somebody died... It does not mean there (are) criminal charges looming or pending or anything like that."

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The death toll meanwhile was likely to go up when authorities extracted and identified the remains of victims in vehicles trapped under the rubble, Perez said.

Ten people were taken to hospital after the bridge collapse, Zabaleta said.

Video footage showed the concrete structure suddenly crashing onto the road below.

Police detective Juan Carlos Llera said when the bridge came down, it "sounded like an explosion. A huge bang."

"It looks like a disaster area. It looks literally like a bomb went off," Llera told AFP.

Miami Fire Chief Dave Downey emphasized there was no hope of finding survivors.

- Loose cables -

The bridge was suspended from cables which came loose, and while they were being tightened the whole thing collapsed, Florida Senator Marco Rubio wrote on Twitter.

The university had just celebrated the walkway's construction, which bridges a busy and dangerous section of highway that students said had been the scene of accidents.

It was raised using an accelerated modular building method that enabled the bridge to go up in the space of a day.

FIGG Engineering Group, one of the partners involved in the walkway's construction, said it was "stunned" by the bridge collapse, and vowed in a statement to "fully cooperate with every appropriate authority in reviewing what happened and why."

Munilla Construction Management, which was also involved, issued a statement of condolence. Bridge collapses in the United States are rare despite rising risks associated with aging infrastructure.

The deadliest such incident this century was in Minneapolis, Minnesota in 2007, when an eight-lane bridge collapsed into the Mississippi River, killing 13 people.