A predawn fire that killed a 30-year-old man and his three children and hospitalized the mother was caused by empty cardboard pizza boxes stored too close to a wood-burning stove, state fire officials said Monday. The family probably used the boxes to help start the fire in the stove after they returned to their Orrington home from bowling late Friday night, fire investigators said. A container of lighter fluid found nearby probably accelerated the spread of the fire once the boxes stored outside the stove ignited and family members had gone to bed. Fire Marshal Joe Thomas called the fire “purely accidental.” It was the deadliest fire in Maine in 20 years. Neighbors reported the fire about 2:30 a.m. Saturday after hearing Christine Johnson, 31, screaming from atop a roof above a breezeway between the house and a garage. Johnson remained hospitalized for smoke inhalation on Monday. The fire killed her husband, Benjamin Johnson III, and their children: two boys, 9-year-old Ben and 4-year-old Ryan, and one girl, 8-year-old Leslie.