Fire investigators have determined that a Plymouth house fire was caused either by an electrical malfunction or smoking by the 77-year-old homeowner, who remains in critical condition with severe burns.

Jennifer Mieth, spokeswoman for the state fire marshal’s office, said the fire, which occurred late Tuesday afternoon at 151 South Meadow Road, has been ruled accidental. She said investigators determined that it started next to a living room chair and are “fairly certain” that it was electrical.

However, she added that they “can’t rule out smoking.”

Because of insufficient evidence, the determination – though inconclusive – is final, she said.

The fire was reported just before 5 p.m. Firefighters found Delia A. Sutcliffe unconscious and with extensive burns on her upper torso. She was flown to Brigham and Women’s Hospital in Boston.

Dorothy Urban, a friend who lives across the street, said Sutcliffe has lived in the home for 15 years and lives alone. She was a nurse in the U.S. Army until she retired as a lieutenant colonel, Urban said.

John P. Kelly may be reached at jkelly@ledger.com.