TULSA, Oklahoma - Tulsa firefighters are working to figure out what caused a shed fire Thursday near downtown Tulsa, killing two people.

The homeowner's family tells news on 6 he believes the victims were squatters.

The medical examiner has not released the identity of the two people who died, but they were adults apparently living in an uninsulated shed, apparently using a hot plate for heat.

The fire didn't go beyond the small shed, but as soon as it was out, firefighters discovered two bodies inside, a man and woman who appeared to be living there.

Police were called, but firefighters believe it started accidentally, possibly when bedding got too close to a makeshift heater.

"Probably not a good idea to be sleeping in a shed to begin with but they were staying warm somehow and I'm pretty confident there wasn't a furnace in there," said Tulsa Fire Department Captain Stan May.

When calling to report the fire, witness Leon Gormin says he didn't realize anyone was inside.

"Wow, dang, I didn't know. I figured someone was staying there when I saw the clothes on the porch, but I didn't think they was in there," Gormin said. 

Firefighters dismantled part of the shed to make sure it was cooled off, while they looked for evidence of what started the fire.

They believe a hot plate could have been the ignition source, but they're continuing to investigate.

The shed had two mattresses inside, two dressers, a microwave, and boxes of clothes, but the homeowner's nephew said they didn't know anyone was living in the shed and the house was rented out.

"Nobody is supposed to be back here, that's why I came out here to make sure it wasn't anybody in our family and it's not," said the homeowner’s nephew. 

The medical examiner took the bodies away for an autopsy and identification.

The two victims were both adults, a man and woman who were middle aged, but authorities have not released their identification yet.