Fire investigators suspect a cigarette or errant spark caused a fire that killed four brothers in Missouri.

A report from the state fire marshal's office shows that the fire started in April after the boy's mother and her boyfriend invited several friends over for a cookout in Lake Ozark. KRCG-TV reports that they used a barbecue grill and later lit a fire pit to keep warm.

The mother's boyfriend told investigators he threw a pitcher of water over two still-burning logs before they left for a bar, leaving 14-year-old Tyler Otto to watch his brothers. The fire killed Tyler, along with 8-year-old Cason, 5-year-old Max and 4-year-old Levi.

Investigators couldn't conclusively determine a cause but narrowed it down to a discarded cigarette or sparks from the fire pit or grill.