DALLAS (AP) — Four people have been found dead in a house under construction in Dallas.

The Dallas Morning News reports that authorities say the two men and two children are believed to have died from carbon monoxide poisoning. Dallas Fire-Rescue spokesman Jason Evans says a generator powering a heater and cellphones was inside the house and had run long enough for the carbon monoxide to reach fatal levels.

Fire officials and police were called to the house in Dallas’ Oak Cliff neighborhood about 9:30 a.m. Sunday.

The victims’ names and ages haven’t been released, but Evans says the children were young boys.

Property owner Hector Medrano says the men were construction workers who recently had to move from their apartments. He says he let them sleep in the home overnight.