An explosion and fire at a Woodstock apartment building that left two people dead was arson, but not intentional, documents show.

The Ontario Fire Marshal’s report into the 2011 Victoria Street explosion, which was obtained by CTV News under freedom of information legislation, lists the fire’s cause as a “negligent arson.”

According to the report, the fire was caused by pentane gases in the building’s laundry room.

A highly flammable solvent, pentane is believed to have been released into the air after plastic substances were melted in a dryer.

It is believed between one and two liters of pentane would have been necessary to cause the explosion in question.

How the plastic ended up in the dryer remains an open question, but officials don’t believe it was done deliberately to cause an explosion.

“There is no evidence it was an intentional act,” Woodstock Police Chief Rod Freeman tells CTV News.

Approximately 65 people were displaced as a result of the March 2011 explosion and fire.

Margaret Gillett, 73, and Bill Watmough, 79, were found dead, buried in rubble under the apartment building, several days later.

A new apartment building has since been rebuilt on the same property.