Such adjustments, which engineers call “post-tensioning,” are common in concrete designs to fine-tune the structure once it is in place. In this case, however, it was not clear whether the cable-tightening was routine or an urgent undertaking in response to the discovery of the crack in the bridge.

Around the same time the meeting was starting, about 9 a.m., Jorge Mesa, a 31-year-old university employee, said he was in his car near the bridge when he heard “a cracking-whip kind of sound.” He looked to his right and saw one of the construction workers on the street.

The worker’s face, Mr. Mesa said, seemed to say, “Um, that’s not normal.”

“When he gave me that face, I got the chills all the way down my body,” he said.

The bridge came down that afternoon.

Witnesses said the collapse appeared to start near the north end. But no one, including the N.T.S.B., has so far placed any blame for the collapse on the cables or cable-tightening work.

By late Saturday evening, recovery crews had extracted four crushed vehicles from under the rubble. Workers covered the vehicles in black sheeting and towed them to the medical examiner to identify the remains of any victims inside.

At a news conference around 10 p.m., Juan J. Perez, the director of the Miami-Dade Police Department, said five people had been found under the 950 tons of rubble. A sixth person, Navarro Brown, was part of the crew working on the bridge and died at a hospital.

“We’re pretty confident that no one’s left,” he said.

The Police Department identified one victim in one of the first vehicles as Roland Fraga Hernandez, and two people in the second vehicle, Oswald Gonzalez, 57, and Alberto Arias, 53.