An engineer left a voicemail alerting officials to cracking in the Miami bridge that left six people dead when it collapsed without warning.

The alert came two days before the catastrophic structural failure and mentioned flaws observed at one end of the doomed concrete span.

But Florida Department of Transportation officials admitted the message was not picked up until after the collapse.

The voicemail, left on a landline, wasn’t heard by the state DOT employee for whom it was intended until Friday because he was out of the office on an assignment, the agency said in an email.

In a transcript released Friday night, FIGG Bridge Group’s Denney Pate said the cracking would need repairs “but from a safety perspective we don’t see that there’s any issue there, so we’re not concerned about it from that perspective”.

The bridge collapsed Thursday, killing at least six people and injuring 10 others. Authorities removing the debris and looking for more victims with the help of sniffer dogs say there is no longer any hope of life beneath the rubble.

“Unfortunately, this has turned from a rescue to a recovery operation,” a Miami-Dade police spokesman said on Friday morning.

The number of the dead could rise as more vehicles could still be under the concrete and twisted metal, he added.

At one point, police requested television helicopters leave the area so rescuers could hear for any sounds of people crying for help from beneath the collapsed structure, CBS Miami television said.

Uncertainty about the stability of remaining sections of the bridge is hampering search efforts.

“The structure is very fragile, it’s very dangerous for rescuers,” a police spokesman said. “The entire bridge is in jeopardy.”

Witnesses told local media the vehicles were stopped at a traffic light when the bridge collapsed on top of them about 1.30pm on Thursday local time

At a news conference Friday night, officials from the National Transportation Safety Board said they had just begun their investigation, and could not yet say whether any cracking contributed to the collapse.

They also said workers were trying to strengthen a diagonal member on the pedestrian bridge at Florida International University when it collapsed.

Robert Accetta, the lead investigator for the NTSB, said crews were applying post-tensioning force, but investigators weren’t sure if that is what caused the bridge to fall.

In a news release late Friday, FIGG Bridge Engineers said it “continues to work diligently” to determine the cause of the collapse, and is examining the steps its team has taken.

It added: “The evaluation was based on the best available information at that time and indicated that there were no safety issues.”

It also asked for time to accurately determine what led to the accident.

Miracle escape

A college student who narrowly escaped from a car that was smashed in the collapse said he watched helplessly as the structure tumbled down atop the vehicle and killed his friend Alexa Duran, who was sitting next to him in the driver’s seat.

Richie Humble, who studies at FIU, was riding in a car under the pedestrian bridge when he heard a long creaking noise coming from the structure which spanned a busy Miami-area highway. It sounded different from anything he had ever heard before.

“I looked up, and in an instant, the bridge was collapsing on us completely. It was too quick to do anything about it,” Mr Humble said Friday in a phone interview with The Associated Press.

Once Mr Humble realized he was alive, he also grasped that he could not get to Ms Duran. He called to her but got no response. A group of men outside the car started yelling at him to try crawling through the rear window.

He couldn’t squeeze through because the window was crushed. The men outside grabbed a wooden plank and pried open the rear door to pull him free, he said.

“I was trying to get people to realise my friend was still in there,” he said.

He suffered cuts to his leg from glass and a slight fracture to a vertebra, but he was able to walk away from the scene.

While families waited for word on their loved ones, investigators sought to understand why the 950-tonne bridge gave way during construction.

Hunting for clues

The cables supporting the span were being tightened following a “stress test” when it collapsed, authorities said.

Scheduled to open in 2019, the bridge would have provided safe passage over a canal and six lanes of traffic. It was conceived and designed as a showpiece architectural feature connecting the FIU campus with the community of Sweetwater, where many students live.

The $14.2 million project was supposed to take advantage of a faster, cheaper and safer method of bridge-building promoted by the university.

Authorities have not confirmed the victims’ names, but Ms Duran’s family revealed she is among the dead. The fatalities are known to include at least one FIU student.

One person died at a hospital and Miami-Dade Police Director Juan Perez said five bodies were located with the help of cameras but had not yet been retrieved.

In a Facebook post, Chelsea Brownfield said she was awaiting any information about her husband, Brandon, who was driving home from work when the collapse happened and has not been seen since.

“The outpouring of love we have received is incredible,” Ms Brownfield wrote.

She declined to comment in a message to The Associated Press.

The bridge was put in place March 10, five days before the collapse.
When finished, the span would have been supported from above, with a tall, off-centre tower and cables attached to the walkway.

That tower had not yet been installed, and it was unclear what builders were using as temporary supports.

-with AP