Transportation Department Skagit River Bridge websiteMOUNT VERNON — Work is on track for the Interstate 5 bridge over the Skagit River to reopen in mid-month, the Transportation Department said Monday.The National Transportation Safety Board has not yet finished its site investigation, but a contractor has started assembling a temporary span that can be rolled into position to replace the collapsed section, DOT spokesman Travis Phelps said.Clearing and investigating debris from the bottom of the river is slow-going.“Portions of the bridge are buried in about four feet of mud that’s under the bridge that’s under water,” Phelps said. Investigators are documenting components as they are removed from the river, NTSB spokesman Peter Knudson said Monday in an email.A 160-foot section of the bridge crumbled May 23 when a truck carrying an oversize load hit a girder. The truck made it off the bridge but two vehicles went into the water and three people were rescued.An NTSB investigator traveled to the headquarters of Mullen Trucking in Aldersyde, Alberta, to gather information about the truck that was carrying the oversize load, the NTSB said.If the company is negligent, the state could make a claim to recover damages, Phelps said. The temporary span and a replacement this fall will cost about $15 million. The federal government is paying most of that.Traffic is detoured through Mount Vernon and Burlington until a temporary bridge is in place. The span carried an average of 71,000 vehicles a day on the main route between Seattle and Vancouver, British Columbia.