Search and rescue teams are carefully unearthing bodies and vehicles from the under the concrete bridge that collapsed suddenly in Miami on Thursday afternoon. And questions over the the bridge's design and construction are just beginning.
The 174-foot, 950-ton main span had been lifted into place just five days earlier. At least six people were killed and 10 were injured in the collapse, according to the Associated Press.
Thursday night, Florida Sen. Marco Rubio tweeted: "The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today."
Because the bridge's central tower and suspension cables were not yet installed, the Miami Herald notes that any cables being tightened were likely wires that ran through the span that fell on Thursday.
The bridge was supposed to make the crossing of a perilous seven-lane roadway safer for students and staff at Florida International University. The bridge connected the university campus to the small suburban city of Sweetwater, where many students and faculty live. An FIU student was killed crossing the road in August.
Rubio is an adjunct professor at the university. His spokeswoman told the newspaper that he had heard this information from workers on site.
At a press conference on Friday morning, assembled authorities would not confirm Rubio's statement.
"This is a tragedy we don't want to re-occur anywhere in the U.S.," said Juan Perez, director of the Miami-Dade Police Department. "Right now we just want to find out what occurred, what caused this collapse to occur and people to die."
Representatives from the National Transportation Safety Board and the Occupational Safety and Health Administration are investigating.
"The people of South Florida have been though a lot, obviously, over the last several weeks and this is just yet one more tragedy to add to that sad book," said NTSB Chairman Robert Sumwalt.
Authorities said their investigation will take time, but early questions have emerged about whether it was wise to install the span across the road without first installing the central support tower.
Last weekend, FIU tweeted a photo of what the completed bridge would look like.
As USA Today reports, the design was for a type of suspension bridge called a cable-stayed bridge:
"Cable-stayed bridges have cables attached directly from the column to the span, while suspension bridges string cables between towers and have other cables descend to the span.
"Amjad Aref, a professor at University of Buffalo's Department of Civil, Structural and Environmental Engineering, said a suspended bridge is typically built gradually, with the center tower or towers erected early.
"Pictures from the scene of the collapse don't show a central tower.
"'Whoever is going to investigate, they will ask the fundamental question: shouldn't the tower be there, and the cables ready to connect to the structure, when you lift it?' Aref said. 'That's a question for them to answer.'"
FIU offers an entire program around bridge projects like this one: the Center for Accelerated Bridge Construction, which opened in 2011. "This method of construction reduces potential risks to workers, commuters and pedestrians and minimizes traffic interruptions," the university said in a press release when the span was swung into space last weekend.
Much of the funding for the $14.2 million bridge and its plazas and walkways came from a U.S. Department of Transportation TIGER grant.
The bridge was designed by FIGG Bridge Engineers and built by MCM Construction. Both companies have been involved in bridge collapses before, as the AP reports:
"FIGG was fined in 2012 after a section of a bridge it was building in Virginia crashed onto railroad tracks and injured several workers, according to a story in The Virginian-Pilot.
"MCM, meanwhile, was accused of substandard work in a lawsuit filed this month by a worker injured when a makeshift bridge MCM built at Fort Lauderdale International Airport collapsed under his weight. Another dispute resulted in a $143,000 judgment against MCM over an 'arguable collapse' at a Miami-Dade bridge project.
"A review of OSHA records, meanwhile, shows MCM has been fined for 11 safety violations in the past five years totaling more than $50,000 after complaints involving its Florida work sites.
Both companies expressed condolences for the victims and promised cooperation with investigators."