SACO (AP) – A propane leak was the likely cause of an explosion at a downtown apartment that sent three people to the hospital, investigators said Wednesday.

Each of the building’s four units had individual propane heaters and the explosion occurred in a vacant apartment on the second floor, said Sgt. Kenneth Grimes of the state fire marshal’s office.

The blast late Tuesday shook the neighborhood and shattered windows within 500 feet, but no one was killed and injuries were not life-threatening, authorities said.

The injured included two of the building’s occupants and one visitor, according to Stephen McCausland, spokesman for the state Public Safety Department.

Firefighters rescued a woman who was trapped inside what was left of the second floor of the building. Another occupant made it out on his own and a third was outside the at the time of the explosion.

The building is on Elm Street, also U.S. Route 1, the main street in the southern Maine city of 17,500. The road was clear Wednesday.