As reports emerged about an engineer's warning of cracks days before Thursday's calamitous collapse of a pedestrian bridge in Florida, a U.S. senator wants to find out exactly who was behind the construction that may have led to the structure's fatal failure.

Sen. Bill Nelson, D-Fla., ranking member of the Senate Committee on Commerce, Science and Transportation, is “demanding” documents related to oversight of the construction of the bridge at Florida International University in Miami, according to a statement from his office.

“There should have been adequate and appropriate oversight on the ground," Nelson writes in a letter to Transportation Secretary Elaine Chao. "If anyone dropped the ball and it contributed to this tragedy, then they should be held accountable.

“There should have been adequate and appropriate oversight on the ground. If anyone dropped the ball and it contributed to this tragedy, then they should be held accountable." — U.S. Sen. Bill Nelson, D-Fla.

“The fact that there were multiple agencies and companies involved, we’re going to need a clear understanding of who had what role in this horrible tragedy.”

FIRST FLORIDA BRIDGE COLLAPSE VICTIM IDENTIFIED AS FIU STUDENT, 18

Even as the scattered wreckage of the bridge was still being searched, officials from multiple offices were looking for answers about the collapse, which killed six people and injured at least 10 others.

Florida’s Department of Transportation said Friday that an engineer left a voicemail just two days before the bridge failure, notifying the department of cracking in the concrete.

A transcript released Friday shows Denney Pate with FIGG Bridge Group saying the cracking would need repairs "but from a safety perspective we don't see that there's any issue there so we're not concerned about it from that perspective.”

The full voicemail can be found below:

"Hey Tom, this is Denney Pate with FIGG bridge engineers. Calling to, uh, share with you some information about the FIU pedestrian bridge and some cracking that's been observed on the north end of the span, the pylon end of that span we moved this weekend. Um, so, uh, we've taken a look at it and, uh, obviously some repairs or whatever will have to be done but from a safety perspective we don't see that there's any issue there so we're not concerned about it from that perspective although obviously the cracking is not good and something's going to have to be, ya know, done to repair that. At any rate, I wanted to chat with you about that because I suspect at some point that's gonna get to your desk. So, uh, at any rate, call me back when you can. Thank you. Bye."

The National Transportation Safety Board announced at a news conference late Friday that it would be launching its own, independent investigation into the bridge failure to try and find out what may have led to the collapse.

NTSB Chairman Robert Sumwalt emphasized “that the NTSB is here to conduct a safety investigation … we are looking at one thing and that is safety.”

“What we are here to do is very simple – find out what happened so that we can keep it from happening again,” he added.

"What we are here to do is very simple – find out what happened so that we can keep it from happening again." — Robert Sumwalt, chairman, National Transportation Safety Board

When asked by a reporter what the protocol would be after finding a crack, Sumwalt said, “I don’t think we know at this point, factually, that there was crack in the bridge."

“But it’s been confirmed,” the reporter fired back.

“Well we have not confirmed that,” Sumwalt answered.

FIU BRIDGE COLLAPSES; 6 KILLED, 10 INJURED, OFFICIALS SAY

In his pursuit of answers about the bridge collapse, Nelson is asking that the federal Department of Transportation turn over “all records related to the engineering, design, construction, safety and inspection of the project.”

Nelson is also asking the federal DOT to provide information regarding a federal grant that was given toward the construction of the bridge, including “all relevant documentation … including the grant agreement itself, any memoranda of understanding between DOT and state and local entities concerning the administration and oversight of this project and any letters issued by DOT related to the project, such as letters of concurrence for the selection of contractors and any letters of approval.”

Nelson is giving the DOT one week to comply with his request.

The $14.2 million pedestrian bridge was set to open early next year to help connect students from an area where many students live to the campus across a busy road. Accelerated construction led the bridge to be installed last weekend.

On Friday evening, FIGG Bridge Engineers released the following statement:

“FIGG Bridge Engineers, Inc., continues to work diligently with the construction team to help determine the cause of the collapse of the pedestrian bridge that was under construction at Florida International University. We are heartbroken by the loss of life and injuries, and are carefully examining the steps that our team has taken in the interest of our overarching concern for public safety. The evaluation was based on the best available information at that time and indicated that there were no safety issues. We will pursue answers to find out what factors led to this tragic situation, but it is important that the agencies responsible for investigating this devastating situation are given the appropriate time in order to accurately identify what factors led to the accident during construction. We are committed to working with all appropriate authorities throughout this process.”

The Associated Press contributed to this report.