BOXHOLM, Iowa (AP) — The Boxholm fire chief says a space heater may be the cause of a fire that killed a mother and her three children.

Boxholm Fire Chief Dave Huffman said the state fire marshal is investigating the Sunday house fire but a preliminary investigation reveals that a space heater may have been the source of the blaze.

The fire was reported around 12:45 a.m. Sunday when the temperature was 6 degrees below zero. Rick Lampe with the Boone County Sheriff's Department said 27-year old Amber Sorenson, 9-year-old Riley Sorenson, 6-year-old Autumn Sorenson and 4-year-old Brayden Sorenson all died in the fire.

Boone County Emergency Management Coordinator Dave Morlan said that firefighters had problems with their gear because of the cold weather.