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 notesthat 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.Witnesses told WLRN the span fell without warning as a traffic light went red. Two workers were on the bridge when it collapsed, according to witnesses interviewed by the Miami Herald.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: