Propane vapor "ignited by an undetermined ignition source" caused the explosion and fire in March that left two Student Transportation of America workers badly hurt, according to an update from the New Hampshire Division of Fire Safety.
The propane came from one of the propane-powered school buses that was being worked on that day, investigators said. However, it's not clear what caused it to explode.
"There were several mechanical and electrical ignition sources discovered inside the facility that could have caused the explosion, but the exact one could not be identified," read the statement from Fire Marshal Sean P. Toomey, Bradford Fire Chief Bryan Nowell, and Bradford Police Chief Edward Shaughnessy. "At this time, there is no indication that this was criminal in nature and the cause appears to be accidental."
The employees injured in the blast have since been released from the hospital and are recovering at home, according to the statement.