ROME (AP) — A two-story building collapsed in the southern Italian province of Naples early Sunday, killing two young siblings and their mother, while burying the children's grandmother under the rubble, firefighters and local authorities said.

The firefighters said on their official Telegram channel that a gas explosion likely caused the collapse.

Rescuers in the town of Saviano recovered first the bodies of a 6-year-old boy and a 4-year-old girl, and a few hours later recovered the body of another victim, identified as their mother, local authorities said.

Searches are still underway to find the older woman, identified as the grandmother of the children, who was believed to be trapped under the rubble.

Earlier on Sunday, the siblings' 2-year-old brother had been found alive with the father, who was hospitalized in Naples in serious condition.

Firefighters’ spokesman Luca Cari said earlier that rescuers had to be “very careful and move slowly, to avoid new collapses” as they searched for the two women.

Mayor Vincenzo Simonelli was at the scene and told reporters that the building appeared “crumpled up on itself,” adding it was “a very serious situation.”

Media reports said a gas leak could have caused the explosion that rocked the second floor of the building, which crumbled down, covering the lower floor with rubble.