National Transportation Safety Board investigators identified pieces of the bridge structure Saturday to collect as evidence to “understand the collapse sequence and what caused it,” said NTSB spokesman Christopher O’Neil.Investigators also gathered documents on the span’s design, construction and inspections, he said.

The NTSB’s lead investigator in the collapse, Robert Accetta, has said crews were working at the north end of the span when it fell. They were applying force “designed to strengthen” one of 10 diagonal elements connecting the walkway and an overhead section. Those diagonal pieces, known as members, are “integral parts” of the structure, Accetta said.

“There were two cables that they were working on at that time,” Accetta said Friday evening. “They were internal to that diagonal member.”

He said it is unclear whether the collapse started in that area, and investigators have not determined whether the tightening of those cables “was related to the cracks that they discovered.”

“A crack in a bridge does not necessarily mean that it’s unsafe,” Accetta said.

NTSB Chairman Robert L. Sumwalt III said investigators also “want to look at how the contractors identified risk and mitigated those risks associated with the construction of this bridge.”

Late Friday, Florida officials revealed that a lead engineer for the private contractor had left a voice mail for a Florida Department of Transportation (FDOT) official Tuesday, two days before the bridge fell, warning of “some cracking.”

The engineer, who works for FIGG Bridge Engineers, reported that he did not consider the cracking a safety issue. The state official was out of the office on assignment and did not hear the message until Friday, after the collapse, the department said.

On Wednesday, an FDOT consultant received a call from an employee of Bolton Perez Associates, the firm hired by the FIU team to oversee construction safety, alerting him to a meeting the following day concerning the bridge, FDOT said. The university said FIGG and the construction firm, Munilla Construction Management (MCM), called a Thursday meeting “to discuss a crack that appeared on the structure.”

During that meeting, a state consultant met with members of the project team but was not told of a safety problem, FDOT said. The state had done a preliminary safety review of the bridge’s design but did not oversee construction safety, a state transportation department official said Friday.

An FDOT spokesman referred safety questions to FIU on Friday, saying it was the university’s project.

At the Saturday briefing, FIU President Mark Rosenberg did not elaborate on FIU’s knowledge of the crack.

“We are cooperating fully with the authorities,” Rosenberg said.

Asked whether he believed that reporting the cracking to the state through a voice-mail message represented “due diligence,” Rosenberg declined to comment, citing the ongoing investigation.

Sen. Bill Nelson (D-Fla.) said he was frustrated that state and university officials were “pointing fingers at the other guy.”

“Whose responsibility was it?” Nelson asked. “If there was a two-hour meeting that occurred from 9 to 11, and the bridge collapsed . . . just a few hours later, then somebody needs to be held accountable.”

Police identified four victims as Oswald Gonzalez, 57; Alberto Arias, 53; Navarro Brown; and Rolando Fraga Hernandez.

Brown died at a hospital, police said. Hernandez was in a gold Jeep Cherokee pulled from the rubble at 5:40 a.m. Saturday, and Gonzalez and Arias were in a white Chevrolet extricated at 7 a.m., police said. Police did not release the names of the final two victims Saturday night. Relatives have confirmed the death of Alexa Duran, 18, according to media accounts.

Perez said chaplains were with victims’ families, and workers held moments of silence as vehicles were brought out “so these victims can have some dignity.”

Martin Weil contributed to this report.