Four members of an Albanian family were found dead Wednesday evening in a municipality in Italy’s Lucca province, authorities said, after a suspected carbon monoxide leak in their home.

Preliminary investigations indicate that the leak was caused by a faulty boiler inside the family’s residence on Via Galgani, according to officials.

Italian daily Corriere della Sera identified the victims as Arti Kola, 48, the head of the household; his wife, Jonida Kola, 43; their 22-year-old son, Hajdar Kola; and their 15-year-old daughter, Xhesika Kola.

A fifth person, the victim’s brother, was rescued in critical condition. He had gone to the three-story house after family members failed to respond to repeated phone calls and raised the alarm.

When he arrived with Carabinieri officers, they knocked several times but received no response. The door was then forced open, and responders discovered the four victims lying lifeless inside the home.

Authorities are continuing their investigation into the exact circumstances of the incident.