MIAMI (AP) — An engineer left a voicemail two days before a catastrophic bridge failure in Miami to say some cracking had been found at one end of the concrete span, but the voicemail wasn't picked up until after the collapse, Florida Department of Transportation officials said Friday.

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

In a transcript released Friday night, Denney Pate with FIGG Bridge Group says 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."

At a news conference Friday night, officials from the National Transportation Safety Board said they have just begun their investigation, and cannot yet say whether that 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 investigator-in-charge for the NTSB, said crews were applying post-tensioning force, but investigators aren't sure if that's what caused the bridge to fall.

The bridge collapsed Thursday, killing at least six people. Authorities are slowly removing the debris, looking for more victims.

A college student who narrowly escaped from a car that got smashed by a collapsing bridge said he watched helplessly as the structure tumbled down on top of the vehicle and killed the friend 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 that 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," Humble said Friday in a phone interview with The Associated Press.

Rescuers are looking for the young woman who was at the wheel, Alexa Duran, whose family said she was dead. Once he realized he was alive, Humble also realized that he could not get to 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 made his way into the back seat but 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 realize 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-ton bridge gave way during construction. The cables supporting the span were being tightened following a "stress test" when it collapsed, authorities said.

"This is a tragedy that we don't want to re-occur anywhere in the United States," said Juan Perez, director of the Miami-Dade police. "We just want to find out what caused this collapse to occur and people to die."

Detectives declared the rubble a homicide scene, and the NTSB arrived to investigate.

Scheduled to open in 2019, the bridge would have provided safe passage over a canal and 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 $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. The fatalities included a student at FIU. One person died at a hospital, and 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. According to a Go Fund Me page set up for the family, Brandon Brownfield was driving home from work when the collapse happened.

"The outpouring of love we have received is incredible," Chelsea Brownfield wrote. "I know you are all concerned for us. We still have not received any news or updates about Brandon Brownfield or the progress of the search (and) rescue."

The post ended with the hashtag "praying for a miracle."

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

Jorge and Carol Fraga feared their relative's car was trapped beneath the bridge. Jorge's 60-year-old uncle, Rolando Fraga, lives in the area and frequently takes the nearby turnpike to work, but no one has heard from him since midday Thursday.

"The waiting is so ... I don't have words for that," Carol Fraga said through tears.

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-center 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.

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Associated Press writers Tim Reynolds, Josh Replogle, Freida Frisaro and Curt Anderson in Miami; Jason Dearen in Gainesville; Tamara Lush in St. Petersburg and Rodrique Ngowi in Boston contributed to this report.