MIAMI: The engineer overseeing a newly built footbridge that crumbled onto a Miami roadway, killing at least six people, had called the state Transportation Department two days before the collapse to report cracks found in the span, the agency said on Friday.

But the engineer's message, including his assertion that the cracking posed no safety issue, was left on the answering machine of a department employee who was away from the office at the time, and it was not retrieved until Friday, a day after the tragedy, according to the agency.

The 950-ton, $14.2 million pedestrian bridge, which crossed an eight-lane highway adjacent to Florida International University (FIU) to link the campus with the city of Sweetwater, collapsed on Thursday, crushing vehicles in traffic below.

Late on Friday, the Florida Department of Transportation (FDOT) released a transcript of a voice-mail message the agency said was left by Denney Pate, senior vice president and principal bridge engineer for private contractor FIGG.

In it, Pate said his team had observed "some cracking" at one end of the bridge and that repairs were warranted, "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."

He added: "Obviously the cracking is not good and something's going to have to be, 'ya know, done to repair that."

The transportation agency described Pate as "FIGG's lead engineer responsible for the FIU pedestrian bridge project" and part of the walkway's "design build team." He did not immediately respond to email queries from Reuters seeking comment on the matter.

The disclosure came hours after US Senator Bill Nelson of Florida demanded documents related to the design, construction, safety and inspection of the fallen bridge while federal and local investigators worked to determine the cause.

"If anyone dropped the ball and it contributed to this tragedy, then they should be held accountable," Nelson, the ranking Democratic member of the Senate Committee on Commerce, Science and Transportation, wrote to the FDOT.

At least six people were confirmed killed, and police have said more bodies may be recovered from the rubble. The accident occurred just five days after the 174-foot-long (53-meter)bridge, assembled by the side of the highway, was installed during a six-hour operation.

"We know that there's people missing, the family members know that there's people missing, and what we can tell them is that we can assume that they're in there," Juan Perez, director of the Miami-Dade Police Department, said at a news conference.

The victims have not been publicly identified, but at least one was a female university student, officials said.

At least 10 people were taken to hospitals; two remained in critical condition on Friday, officials and local news media reported.

Uncertainty over the stability of remaining sections of the bridge hampered rescue efforts, officials said.

Some news media accounts reported that engineers may have been conducting a stress test that might have led to the collapse. But the state Transportation Department said it had no knowledge of any such tests being scheduled since the bridge was installed last Saturday.

It was too early to say whether anyone might face criminal charges, Perez said.

National Transportation Safety Board officials were on the scene on Friday to investigate.

Munilla Construction Management (MCM), which installed the bridge, said it was devastated by what happened, was cooperating with investigators and doing everything it could to help.

The Miami-based company, which also has operations in Texas and Panama, employs about 500 people and specialises in civil projects, airports and educational facilities. Since being founded in 1983, it has handled billions of dollars worth of projects in Panama, Florida and the US Southeast.

MCM appeared to have the backing of Miami-Dade County to build a planned $800 million bridge between Miami and Miami Beach, even receiving the county's support in a lawsuit seeking to block Florida officials from awarding it to a competitor.

According to campaign finance reports, the company and the five brothers who own it give generously to candidates at the local, state and federal level. MCM officials did not respond to requests for further comment on Friday.

The Florida Transportation Department said on Friday that one of its consultants on the project attended a meeting with members of the bridge design and construction team shortly before the walkway failed and was "not notified of any life-safety issues."