BRADFORD, N.H. — An explosion and fire at a school bus transportation center that seriously injured two workers was caused by vapors released from a propane-powered bus that ignited, the state Fire Marshal’s Office said Wednesday.

The two men who suffered serious burns were working on the bus at the Student Transportation of America building in Bradford in March.

“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,” New Hampshire State Fire Marshal Sean Toomey said in a news release.

Toomey said at this time, “there is no indication that this was criminal in nature and the cause appears to be accidental.”

He said the two workers have been since been released from the hospital and are recovering at home.

The buses were used by the Kearsarge Regional School District.