The National Transportation Safety Board has issued its preliminary report into the May 23 bridge collapse along Interstate 5 in Washington, sending two vehicles into the Skagit River below.

It says immediately prior to the collapse, a 2010 Kenworth truck-tractor in combination with a 1997 flatbed trailer with and oversize load was following a pilot vehicle traveling southbound on Interstate 5.

According to witnesses, as both vehicles approached the bridge in Mount Vernon at mile marker 228, another southbound tractor-trailer overtook and passed the oversize load in the left lane. The driver of the oversize load reported to investigators that he felt “crowded” by the passing combination vehicle so he moved his vehicle to the right.

As the oversize load was being transported across the bridge, the top of the load collided with the overhead portal and multiple sway braces on the far right side of the truss structure. The impacts, says, NTSB, caused significant damage to load-bearing members of the bridge’s superstructure, resulting in the failure and subsequent collapse of the northernmost bridge span.

During the post-collision investigation, the driver reported to investigators that he thought the height of the oversize load was 15 feet 9 inches. The lowest portion of the sway braces, as measured over the active portion of the roadway, was determined to be 14 feet 8 inches. According to the operator of the pilot vehicle, the clearance pole mounted on the front of her vehicle was set at 16 feet 2 inches.

The bridge, constructed in 1955, had four concrete approach spans on the north and south ends and four 160-foot-long steel through-truss spans over the river. The collapsed span, located on the north end of the truss portion of the bridge, consisted of two northbound and two southbound traffic lanes divided by a concrete barrier.

Two passenger vehicles were on the bridge span at the time of the collapse. The vehicles and the damaged span fell into the river. The three vehicle occupants were later rescued from the water. The motorists on the collapsed span received injuries of varying degrees; no fatalities resulted from the collapse.

In addition to the span that collapsed, at least one adjacent span was found to have impact damage from the oversize load.

Shortly after the bridge collapse, the Washington State Department of Transportation began work on building a temporary span to replace the part the I-5 bridge that broke away. It is scheduled to open sometime next week.