NORTH ADAMS — The cause of a large fire at a State Street junkyard last month has been ruled accidental, the State Fire Marshal's Office said Wednesday. 

The fire at George Apkin & Sons on May 25 ignited as a result of heavy equipment being used on a car that still had gasoline in the tank, according to a news release.

"The gasoline vapors ignited, causing the fire," the release stated. "The operation was routine in terms of what the junkyard does daily."

The fire sent black smoke billowing over the city and forced residents to shelter inside their homes because of air quality concerns. Fire crews from around the region were called in to assist, and the blaze took nearly 24 hours to extinguish.

The fire was investigated by the North Adams fire and police departments and State Police assigned to the State Fire Marshal's Office.