WEST ALLIS, WI (WTAQ) - State and federal investigators are still trying to figure out what started a duplex fire in West Allis that killed two teenagers early Sunday.

Officials confirmed Monday that no adults were in the upstairs unit when Michael Gonzales and Isaiha Kobow, both 14, were killed. Two younger Gonzales brothers are expected to survive.

West Allis fire lieutenant Jay Scharfenberg said the entire second floor was burning, while a number of firefighters tried to reach an attic where the youngsters were. He said they kept pushing back the flames – but the fire came roaring back, and the ceiling collapsed on them.

Officials said a man and a woman lived downstairs. The man heard a smoke alarm and smelled something from upstairs – and as he kicked open the door, the two young survivors were just leaving.

Officials said it was the most intense house fire that many firefighters had ever experienced.

Units from Brookfield, Wauwatosa, and Greenfield helped West Allis firefighters put it out.