SACO – 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 of buildings 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.

Alfred Wilson, who rented one of the building’s four apartments, said he was lucky to be alive.