The National Transportation Safety Board has released an update in its continuing investigation into the May 23 bridge collapse in Mount Vernon, Wash., along Interstate 5.

Its investigators interviewed the driver of the pilot vehicle on Monday. The driver, who investigators characterized as very cooperative, described the approach to the bridge and indicated that she saw it collapse in her rearview mirror. The account of her activities in the 72 hours prior to the accident did not reveal anything unusual or of significant interest to investigators. 

Investigators also examined the pilot vehicle for the first time on Monday. The vehicle was equipped with a fiberglass measuring pole mounted on the front of the pick-up truck, which was used to detect potential vertical clearance obstructions.

The NTSB says it is aware of reports of a second tractor-trailer combination unit on the bridge at the same time that the bridge was struck by the truck pulling the oversized load. Investigators are attempting to track down that driver to gather any additional information that may be relevant to the investigation.

All of the bridge structure components of interest to investigators have been removed from the river and are in the process of being documented. While additional witness interviews may be conducted, the NTSB has concluded its work at the bridge site. 

About a week after the bridge collapse, an NTSB investigator traveled to the headquarters of the company that was pulling the oversize load that struck the bridge, and possibly causing a 160-foot section of the bridge to collapse, Mullen Trucking in Alberta, Canada. The purpose it says is to gather information on the motor carrier’s operations, safety history, training process, maintenance procedures and records, and other documents related to the investigation. 

In a media briefing near the accident site on May 25, the NTSB said that Washington State does not require vertical clearance signage to be posted unless the clearance is fourteen feet four inches or less. Further research indicated that the actual minimum vertical clearance over the travel lanes requiring signage is fifteen feet three inches or less. In either reference, the Skagit River Bridge measures greater than fifteen feet three inches over the travel lanes and therefore does not require signage.

The incident resulted in Mullen earlier blaming the Washington state DOT for allowing the load to use the route while the state has said the ultimate responsibility for making sure the route was proper was the responsibility of the trucking company.

NTSB hasn’t said how long it will take to complete its investigation, but a preliminary report is expected by the end of the month.

There are reports Washington may seek payment from Mullen and its insurance company for the damage, if NTSB determines the collapse was due to negligence by the trucking company.

Traffic is currently being detoured around the collapse leading to some traffic delays, especially heading north around peak traffic times. A temporary bridge is on track to be installed and open around mid-month.