A fire in Winthrop that significantly damaged a home Monday was caused by improper disposal of a cigarette or cigar, according to an investigation by the Office of State Fire Marshal.

The fire, which was reported by a passerby shortly before 3 p.m., began in mulch near or below the porch before spreading to the rest of the house and attached garage, investigators say.

Once bark mulch dries out, it is easily ignited by discarded cigarettes or hot ash, Shannon Moss, public information officer for the Department of Public Safety, said in a news release Thursday. Moss said bark mulch can smolder for a while before open ignition occurs.

The fire, at 124 Route 41, took four hours to extinguish, and two firefighters were transported to MaineGeneral Medical Center in Augusta for heat exhaustion. They were later released.

One resident and three dogs were safely evacuated from the house.

Crews from Manchester, Readfield, Augusta, Monmouth, Wayne, Mount Vernon and Vienna fire departments assisted Winthrop’s department to bring the fire under control, Moss said.