WATERVILLE — An early morning fire that killed a young woman Thursday most likely was caused by careless of disposal of smoking materials, Department of Public Safety spokesman Steve McCausland said.

Waterville Fire Capt. David Lafountain said firefighters rescued 28-year-old Craig Sargent and his 2-year-old son, Matthew. They found 23-year-old Amber Mitchell in a back bedroom.

She was pronounced dead at MaineGeneral Medical Center.

Sargent and his son were taken to St. Mary’s Hospital in Lewiston, where they are being treated for smoke inhalation.

Officials say the fire broke out shortly after 6 a.m. in an upstairs apartment of a two-unit house on Riverview Street.

According to McCausland, investigators said the fire started in the living room and he said a battery-operated smoke detector had been removed and was found in a kitchen drawer.