An engineer left a voicemail warning of cracks in a pedestrian bridge just two days before it collapsed in Miami it emerged, as police declared the rubble a homicide scene.

The voicemail left on a landline was not picked up by a Florida Department of Transportation employee until Friday because they were out of the office on an assignment, the agency said in an email.

Denney Pate, of FIGG Bridge Group, said on the message the cracks would need repairing, “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”, according to a transcript released on Friday.

Work had begun on the bridge just five days before it collapsed near Florida International University (FIU) Thursday, killing at least six people.

Authorities are slowly removing the debris, looking for more victims.

Detectives have now declared the rubble a homicide scene.

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

The cables supporting the span of the bridge were being tightened following a ”stress test“ when it collapsed, it has been revealed – but the DoT said it had not been notified of the test.

A college student, who narrowly escaped from a car that was smashed in the collapse, said he watched helplessly as the structure tumbled down on top of 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 also riding in a car under the pedestrian bridge when he heard a creaking noise coming from the structure that spanned a busy Miami-area highway.

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

Mr Humble said he was unable to get to his friend Alexa, who was unresponsive.

A group of men outside the car had to force open the rear door with a wooden plank and pull him free.

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

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

The fatalities included a student at FIU and one person died at a hospital. 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 information about her husband, Brandon.

According to a GoFundMe page set up for the family, Brandon Brownfield was driving home from work when the collapse happened.

Jorge and Carol Fraga also feared their 60-year-old uncle Rolando Fraga was trapped beneath the bridge in his car.