DES PLAINES, Ill. (AP) — A fire in a suburban Chicago apartment that killed four young girls and their mother may have been caused by a space heater that also could have blocked the family’s only path out of the burning building, fire officials announced Thursday.

The space heater was at the top of stairs that were the only way in and out of the second-floor unit, where all of the family members were found, the Des Plaines Fire Department said in a news release. No smoke detectors were on the second floor.

The investigation is continuing and will include an “engineering analysis” of if and how the heater may have started the blaze, the department said. Foul play is not suspected.

Citahaly Zamoida, 25, and her four young daughters — Renata Espinosa, 6; Genesis Espinosa, 5; Allizon Espinosa, 3; and Grace Espinosa, 1 — were inside the apartment when the fire swept through on Wednesday, killing them all. The children’s father was not home.

The family lived in a 104-year-old structure that had been divided multi-family units, according to the Des Plaines Fire Department.

In its news release, the department said the building has a lengthy history of property maintenance code enforcement violations, including a citation for illegal burning.