Firefighters battled a fierce blaze and exploding propane tanks inside a pop-up camper set up behind a vacant building. No one was injured.

Several people called police at about 9:30 p.m. Tuesday to report the sound of an explosion coming from the property at 324 Howard St.

Rockland Fire Chief Scott Duffey said the 12- by 6-foot camper was engulfed in flames when firefighters arrived. It took them about 20 minutes to extinguish the fire. Complicating their efforts, Duffey said, were exploding propane tanks inside the camper and several sitting near it.

“We had actively leaking propane tanks. We kept the hose lines on them to disperse the propane into the atmosphere and keep the heat down,” he said.

The state Fire Marshal’s office is investigating the cause of the fire. “The usual causes of fire were not present and foul play is being investigated related to this fire,” Duffey said.