Photo by AP Wideworld

One worker died in the first bridge collapse, while the second occurred more than 12 hours later.

The fatal collapse of an under-construction pedestrian bridge and—more than 12 hours later— the overnight crash of another nearby span at a $49-million community-college project in Raleigh, N.C., have contractors, engineers and state safety officials stumped for a cause of the two failures at the Wake Technical Community College Building F project.

On Nov. 13, around 10:30 a.m., workers with Skanska USA Building's subcontractor, Central Concrete of North Carolina, were placing concrete on the deck of a bridge, connecting Building F with a nearby building, when it collapsed.

In an interview with Raleigh TV station WRAL, one of the three injured workers, Jose Hernandez, said the collapse sounded "like a bomb." One worker, Jose Luis Rosales-Nava, was killed.

Contractor and rescue officials immediately closed off the area surrounding both structures, which are located on the site's perimeter in a wooded area. The next morning, contractors discovered that a second bridge had collapsed overnight. Allen Jones, senior vice president with Skanska, said the estimated time of the second collapse was between midnight and 12:30 a.m. This bridge, located in the same general area as the first collapsed span, connects Building F with an under-construction parking deck.

Skanska officials described the bridges as being composed of "glue-laminated parallel beams, with a steel frame and concrete on a metal-deck walking surface."

According to project architect Clark Nexsen's website, the building site features a 14-ft grade change and is positioned near wetlands. Stewart Inc., Raleigh, is the project's engineer, according to the contractor.

Skanska started work on the project in March 2014 and was on schedule with all aspects of its contract, says both the firm and the school. The project's completion remains scheduled for 2015.

"I have no idea" what caused the dual failures, says Jones, adding, "We have our forensic engineers out there, and Stewart has their forensic engineers out there."

Stewart officials would not answer questions from ENR but issued a statement, which read, in part: "We are cooperating fully with the investigation. ... Important findings of fact from the conclusion of the investigation will be shared with the industry."

ENR's calls to Central Concrete of North Carolina were not returned. Architectural firm Clark Nexsen also did not respond to questions posed by ENR. The North Carolina Dept. of Labor's Health and Safety Division is leading the investigation into the accident.