Updated

A fire in a room at the 300-year-old Frances Canyon pueblito didn't burn hot enough to compromise the structure, but soot has stained stones and mortar, a newspaper reported Monday.

Bureau of Land Management officials believe the fire was set Oct. 31 during a Halloween party in the structure, The Daily Times in Farmington said.

Authorities say those responsible blocked a doorway with a log, filled in a hatch with rocks and used a beam as a ladder to reach the roof.

Jack-o'-lanterns, candy wrappers, glow sticks and fake spider webs were found at the ruin, about 50 miles east of Farmington.

"They treated it as though it was a haunted house," said Larry Baker, executive director of Salmon Ruins Museum and the San Juan County Museum Association.

It's a federal crime punishable by jail time and fines to vandalize, damage, unlawfully excavate or loot archaeological sites.

"I think it's possible that these people were oblivious that this was an inappropriate activity," Baker said.

When the vandals climbed onto the roof, they walked on original beams, which could easily break. The museum is now considering adding a sign to tell people to stay off the roof.

"It's visibly intrusive, but, I think, at this point, necessary," Baker said.

While most of the damage has been cleaned up, the fire left a mark that will take years to disappear.

"This is damage," Baker said, pointing to the blackened wall, "this is true vandalism."