of questions tonight about that new bridge, built beforehand and trucked to the scene. Tonight, we have just learned that the NTSB is now investigating. And look at this video tonight, showing workers putting the 174-foot bridge right into place it took them just hours to do that over the weekend last weekend. ABC's David Kerley tonight on this part of the story. Reporter: The remnants of some of the 950 tons of concrete and steel were part of what is known as an "ABC bridge," accelerated bridge construction. Some dub it an instant bridge. In fact, that the pre-built span in a matter of hours over the weekend was moved into place. Traffic was soon traveling below. The idea is to build the bridge span, in this case, on the side of the road, keeping workers and drivers safe, and then, in less than a day, put the span in place with little disruption of traffic. 800 bridges have been built this way, or with pre-built elements, since 2010. The federal transportation department lauds this type of construction, and paid most of the 14 million does for this bridge, which was supposed to withstand a category 5 hurricane and last 100 years. It usually is safer, because they're doing all the construction away from the traveling public. Reporter: Now, federal investigators from the NTSB will try to determine, was it a design flaw, a material or construction issue? Both the bridge architect and the Miami construction firm say they are stunned by the tragic collapse and will fully cooperate with the investigation. So, let's get to David Kerley, live with us tonight. And federal investigators are on the way, and you're learning that the engineering company was actually conducting stress tests on that bridge today? Reporter: Another company, according to sweetwater city officials, an engineering firm doing a stress test in the morning. It is unclear, David, if the test had been completed or was ongoing when the bridge collapsed.