Philadelphia investigators have determined that a fire Saturday in a Mayfair home while a family was inside was arson.

Philadelphia police said a 37-year-old woman was home around 5 p.m. with her two 6-month-old twins when the house began to fill with smoke.

Investigators said that a dog bed in the basement was found on fire. The backdoor was found unlocked, they said.

Investigators ruled the incident an arson after 7 p.m. No arrests have been made.