OKLAHOMA CITY - Investigators still haven't determined what started a fire at the former John Marshall High School. The fire Wednesday night is the latest in a number of fires at old abandoned schools.

Part of the issue, according to firefighters, is with MAPS, there are more new schools and more abandoned ones. And schools, like other abandoned buildings, become perfect places for squatters who start fires.

When firefighters were able to put out the fire Wednesday night at the former John Marshal high school, they quickly found evidence as to what may have started it.

"They had a mattress, maybe a couch, some other debris inside the structure," said Fire Chief Phelton Morgan with the Oklahoma City Fire Department on Wednesday night.

"It is a shame, you know, losing history of any kind is always a shame," said Justin Tyler Moore who runs the web site abandonedok.com, which documents abandoned and historic properties.

Tyler says MAPS left 13 properties abandoned. Nine have been demolished. And abandoned schools, like other abandoned buildings, can become havens for transients.

"The property owners can do their very best to secure these properties, but they're going to get in," said Major Tammy McKinney with the Oklahoma City Fire Department.

McKinney says the cause of most of the school fires is undetermined but they suspect they are started by people who don't belong in the buildings.

"This is a school, it was a school to thousands of kids so when it's gone or something happens it means something," Tyler Moore said.

Still, Major McKinney says the problems go beyond sentimentality.

"From the fire department standpoint, the issue becomes firefighter safety, when we make entry into these buildings we have no idea what's been done to the structure," Major McKinney said.

A spokesperson from Oklahoma City Schools says they have tried to sell some of those old buildings and they have operational procedures in place to address security issues at their vacant buildings.