MOUNTLAKE TERRACE – An apartment fire that killed two young brothers Monday was likely started by the boys playing with a cigarette lighter, fire investigators said Wednesday.

After sorting through the charred apartment, investigators found a lighter inside a large walk-in closet where the boys, ages 4 and 6, were either playing or sleeping, Mountlake Terrace Fire Marshal Steve Sherman said.

A small can of lighter fluid was found outside the closet. The fire started at the closet entrance, blocking off the boys’ only escape route.

Sherman said the boys’ father was asleep in the bedroom and woke up after the fire started. He apparently ran out of the room thinking the boys were outside, Sherman said.

“This is truly a horrible tragedy,” Mountlake Terrace Assistant Police Chief Mike Mitchell said. “It affects the whole family forever, and affected everybody in the police department.”

It’s unclear whether lighter fluid was spilled on the floor or exactly how the fire spread. Sherman said the windows were probably open and the fire had plenty of oxygen and fuel to spread quickly.

The Snohomish County Medical Examiner’s Office has yet to release a cause of death.

The fire raced through the bottom-floor unit of Village Square Apartments in the 5000 block of 212th Street SW Monday and quickly spread to the unit above. The boys were home with their father, grandfather, aunt and 13-month-old cousin at the time of the blaze.

The boys’ father helped the grandfather escape and tried to go back for his sons. He broke through a window but was forced back by the intense heat and smoke.

Fire crews battled the two-alarm fire but were unable to reach the boys. The father had to be physically restrained from going back into the building to rescue his children. He suffered first- and second-degree burns.

After their two-day investigation, officials concluded the fire was caused accidentally by the boys.

Sherman said investigators found a smoke alarm outside the bedroom, but witnesses told them that they did not hear it go off. It’s the responsibility of apartment owners to provide a smoke alarm and the responsibility of tenants to maintain it, Sherman said.

The marshal said the fire department intends to spend time educating the community about fire hazards.

“The public-safety message here is to be accountable for materials like lighters,” Sherman said. “That’s emphasized by this terrible tragedy.”

Herald writer Diana Hefley contributed to this report.

Reporter Chris Collins: 425-339-3436 or ccollins@heraldnet.com.