Authorities say a 6-year-old boy died after he and two siblings, ages 9 and 4, were playing with fire in a “fort” made from a concrete stairway on a flatbed trailer.

The children used a lighter Sunday afternoon to set fire to “a small pile of combustible objects as though it was a campfire,” the Louisiana State Fire Marshal’s Office said in a news release Monday.

They had covered a hollow prefabricated concrete stairway as their play fort. “While the other two children were able to escape unharmed, the 6-year-old was trapped and died,” the statement said.

When Geismar firefighters and Ascension Parish sheriff’s deputies arrived, they found the flatbed and a cargo trailer and its contents ablaze. The boy’s body was found under the cargo trailer’s contents, the news release said.

The trailers were in the front yard of relatives who lived next door to the children’s family, and adults were in both homes at the time, the statement said.