MIAMI — Sgt. Jenna Mendez was heading to work at the Sweetwater Police Department on Thursday when she caught the long red light at Southwest 112th Avenue. She was not happy: She had a 2 p.m. meeting to make.
With the windows rolled up and music on, she waited. Then she saw the brand new bridge about 300 feet in front of her, just installed by Florida International University, suddenly come down. It fell all at once.
Her first thought: Why would construction workers do such a thing?
“Obviously it was a collapse, but I couldn’t comprehend it,” she said on Friday. “I was thinking: ‘Why did they just block all those lanes of traffic?’”
Ahead of her, Alexa Duran and her friend, Richard Humble, both F.I.U. students, had also been waiting out a red light in a Toyota 4Runner.
They were stopped under the bridge.
The concrete overhead creaked. Mr. Humble looked up, and saw the structure falling. It crushed the car and squashed his neck, trapping him inside.
“I thought when I saw the bridge coming down that I was dead,” he told NBC News.
Passers-by freed him. But at least six people did not survive — including, most likely, Ms. Duran, a freshman who lived at home and was close with her Ecuadorean-immigrant parents, often pressing shirts at the family’s dry cleaning business.
By Friday afternoon, she still had not been officially identified as among the dead, but her father, Orlando Duran, feared the worst.
“She was an angel,” Mr. Duran said by phone from London, where he was traveling for work, before he got on a plane back to the United States. “She wanted to become a lawyer, and she was so beautiful.”
On Friday, the day after the collapse, the authorities in Miami-Dade County announced that they had called off the search for survivors and turned their attention to finding out exactly why a new pedestrian bridge — hailed as a breakthrough in speedy, safe construction — had given way over the road beneath, crushing at least half a dozen people to death.
As a backhoe slowly lifted the rubble off cars, some of them with bodies still inside, investigators said they were only beginning to consider possible failures of engineering, construction and traffic planning. Officials from the local police to the National Transportation Safety Board are expected to scrutinize an array of issues, including whether a busy thoroughfare should have been open on a day when the walkway apparently was undergoing crucial checks and adjustments.
“Right now, we just want to find out what occurred, what caused this collapse to occur and people to die,” said Juan J. Perez, the director of the Miami-Dade County police.
The $14 million walkway, which was to carry F.I.U. students and other pedestrians over Southwest Eighth Street, was built using “accelerated construction,” a well-regarded method of erecting bridges that avoids the long months of street closings when a structure is built over a road or river. Instead, parts of the bridge are prefabricated away from the site and then moved into place.
Bridges made using the accelerated techniques are not more at risk of collapse than others, but moving them into place causes different stresses than what the bridge would normally have to withstand, said Andy Herrmann, a former president of the American Society of Civil Engineers.
The bridge was a cable-stayed design, in which the span would have eventually been supported by cables from a tower that were tied directly into the concrete walkway. Usually, the tower is built before the span, but in this case, the tower had not yet been built; the walkway, which had not opened to pedestrians, was resting on concrete piers when it collapsed.
“It’s unusual, on the one hand,” said Michael Stein, managing director with Schlaich Bergermann, an international engineering firm not involved with the project. “But you can obviously do it.”
There was still conflicting information about what work was occurring on the bridge when it fell.
Mark B. Rosenberg, the university president, said that there had been testing underway, without being more specific. Senator Marco Rubio, who lives not far from F.I.U., said on Twitter that the collapse happened as loose cables were being tightened. He learned of that detail from several workers at the site, his office said.
An executive at a construction company that was working on the bridge confirmed on Friday that one of his employees had been killed and two others hospitalized.
Mike Biesiada, the chief sales and marketing officer for the company, Structural Technologies, said the employee who died was Navaro Brown, 37. “We look forward to learning the cause of the accident so that it’s not repeated ever again,” Mr. Biesiada said.
One of the major questions is whether Southwest Eighth Street, one of the Miami area’s most traveled roads, should have been open at the time. Frank Guyamier, deputy director of engineering for Miami-Dade County, which had no jurisdiction over the bridge, said that generally, the decision to close a roadway is made on a case-by-case basis, depending on the work and its potential danger to motorists and pedestrians below.
Most county pedestrian bridges, including one recently completed in front of the University of Miami, are steel structures that, once tied down, do not require any more road closings, he said. The F.I.U. bridge was different, he said: “This was really a signature structure.”
The bridge designers, builders and inspectors were all well-known and influential firms. None responded on Friday to queries about the major questions that have emerged.
Figg Bridge Group, based in Tallahassee, designed the bridge, as well as a number of significant bridges in the United States, including a new span on Interstate 35W in Minneapolis that replaced a section that collapsed in 2007, and the iconic Sunshine Skyway Bridge over the mouth of Tampa Bay.
Figg issued a statement on Friday that called the collapse “truly tragic” and said it was cooperating with the investigations.
It added: “No other bridge designed by FIGG Bridge Engineers has ever experienced such a collapse.” But a 90-ton segment of a bridge across the Elizabeth River in Virginia collapsed during construction in 2012, leaving four workers with minor injuries. The company was fined $9,800 by the state, according to federal records.
The main builder of the F.I.U. bridge, Munilla Construction Management, specializes in roads, bridges and other infrastructure projects, and has a $63.5 million contract with the United States Navy to build a school on the naval base at Guantánamo Bay. It was founded by brothers who were Cuban refugees and whose father, the firm says, owned a construction company in Cuba that was confiscated by Fidel Castro.
The company is known for having close relations with and donating to local politicians. A spokesman on Friday declined to comment on the bridge collapse, saying that “due to N.T.S.B. investigation requirements, we will not be able to address any questions pertaining to this construction project.”
The bridge had been a point of pride for F.I.U., which has a center devoted to accelerated bridge construction and hosted a “watch party” last Saturday when the bridge was moved into place. Dr. Rosenberg, the president, said the university was conducting its own investigation.
“Obviously, everybody is in shock here,” said Dr. Rosenberg, who had been a public champion of the project. “We just want answers, and we’re going to get answers.”
Also seeking answers were some of the families of those missing and presumed dead. Some bodies remained in cars under the rubble on Friday, hampering the ability to officially identify them.
Jorge Fraga said that his uncle, Rolando Fraga, a 60-year-old Cuban immigrant, had been missing since Thursday afternoon, when he told his wife over the phone that he was on his way home for lunch.
“We’re trying to find out what’s going on,” the younger Mr. Fraga said, adding that authorities had given them little information in the last day.
An online fund-raiser for the family of another missing man, Brandon Brownfield, had raised more than $20,000 by Friday evening.
“We have not received word about his whereabouts or his medical condition,” Chelsea Brownfield, Mr. Brownfield’s wife, said in an interview. The fund-raiser page said they have three daughters.
Even those who had escaped injury were still stunned on Friday.
Dania Garlobo, 51, a manicurist, had been driving in her Jeep to her job at a beauty parlor. But as she approached the traffic light just past the bridge, she was cut off by several cars moving into her lane.
“Drivers in Miami have no manners,” she said.
South Florida’s notoriously aggressive drivers, she said, may have saved her life. The hood of the vehicle two cars ahead of her was crushed.
“I stayed in my car and cried and screamed and called 911,” she said. “It was horrible, horrible, horrible.”
Sergeant Mendez, 36, climbed to the top of the fallen bridge, where she found four wounded construction workers.
The only shouting she remembered was her own: “I need rescue! I need doctors!”
She started doing chest compressions on one man who was not breathing. A motorist who said she was a doctor was guided by other drivers up the heap, where she helped administer CPR.
Sergeant Mendez started crawling under the rubble to see if anyone was trapped. She had momentarily forgotten about the grave danger of sliding beneath a collapsed 950-ton bridge.
“Fire rescue started screaming: ‘What are you doing? Do not go under that!’” she said. “Once I stepped back and looked, I realized: ‘There’s nothing I can do, these cars are crushed.’”
She stayed at the scene until 10 p.m., went home and gave her children, ages 21, 17, 13, 10 and 3, extra tight hugs, and took some melatonin to help her sleep.
“I told my husband, ‘I don’t want to drive under a bridge anymore,’” she said.