The busy highway beneath a Miami bridge which collapsed killing six people last year should have been 'completely closed', a top engineer said in newly released documents.

The National Transportation Safety Board (NTSB) released nearly 6,300 pages on Tuesday as part of its examination into contractors tied to the Florida International University bridge which collapsed on March 15, 2018.

Will Watts, of Florida's Department of Transport (FDOT), wrote in a September 20 letter to the NTSB that the road under the bridge should have been 'completely closed to traffic if the contractor was undertaking activities that posed a risk to the public.'

In addition, the NTSB documents showed a firm responsible for evaluating the bridge's structural integrity were not qualified to do so.

The 174-foot-long span weighing 930 tons collapsed onto traffic flowing on the roadway beneath the bridge, killing Brandon Brownfield, 39, Rolando Fraga Hernandez, 60, 57-year-old Oswald Gonzalez, 53-year-old Alberto Arias, 37-year-old Navarro Brown, and FIU student Alexa Duran, 18.

Scroll down for video.

Dashcam footage from the moment the bridge collapsed on March 15, 2018, killing six people

Emergency personnel swarm around the pedestrian bridge after it collapsed onto the highway at Florida International University in Miami on March 15, 2018

A report by the U.S. Occupational Safety and Health Administration had already revealed that the bridge suffered extensive cracking in the days before the collapse because of a 'deficient' structural design.

Engineers had knowledge of extensive cracking and failed to order the closure of the busy highway 11 miles west of downtown Miami, investigators said.

Watts wrote: 'At the core of this issue is sound engineering judgment ... [FDOT] employees faced with a situation like the one presented by the FIU bridge would have been expected to take immediate action to close the road.'

The documents also showed that FDOT listed Louis Berger Group, Inc. as pre-qualified for an evaluation it was to carry out on the bridge but this was a 'technical error' on its website.

In emails between FDOT representatives and an NTSB investigator, the state's transportation department said firms involved in the project should not have simply relied on the website as proof of Louis Berger's credentials, and should have done their own due diligence, such as seeing an actual letter of qualification from the state.

Furthermore, Louis Berger should have known whether it was qualified for the work, FDOT's special counsel Latasha Johnson wrote in an email to an NTSB investigator.

The level of qualification for that project required companies to have at least three professional engineers registered with the Florida State Board of Professional Engineers, and a minimum of five years of structural concrete bridge design experience.

FDOT told NTSB that Louis Berger never received the qualification that pedestrian bridge required. The company lost a qualification it had for less complex bridge designs in December 2016 because they lost some of their engineers.

Navarro Brown was working with VSL Structural Technologies on the pedestrian bridge. FIU student Alexa Duran, 18 (right), was giving her friend a ride to his doctor's office to pick up some medication when the bridge collapsed, killing her

Oswald Gonzalez, 57 (left), and Alberto Arias, 53 (right), friends and business partners, were driving to a travel agency to pick up tickets for their annual visit to their homeland of Cuba

Rolando Fraga Hernandez's (left) body was found when his vehicle was removed from the debris. Brandon Brownfield (right) was the final victim named

Louis Berger did not immediately respond to a request for comment.

The company that designed the bridge, FIGG Bridge Engineers, Inc., said in a response to the NTSB findings that there was no disclosure on the state website saying firms shouldn't rely on its website.

FIGG employees evaluated cracks found on the bridge and said they didn't find safety concerns.

A Federal Highway Administration assessment released with the trove of documents on Tuesday found that FIGG 'made significant errors,' in its design calculations, leading to cracking that wasn't properly addressed.

The project was designed to look like a cable-stayed bridge, with steel pipes branching out from a tall mast. But it was never completed.

Contractors used a method that avoids disrupting traffic by building bridge spans offsite and then transporting them to the location in a matter of hours.

Engineers began noticing cracks soon after the bridge was put in place on March 10, 2018, days before the collapse.

A crew was on the bridge working on tightening a diagonal beam the day it collapsed onto traffic.

NTSB will discuss the findings at an October 22 public board meeting that's intended to determine the probable cause of the collapse.