An explosion at a women's spa Tuesday killed three people and injured at least three others, officials said.

Methane gas appeared to have seeped from the ground beneath the popular "Shiespa" hot spring and ignited, according to police. Authorities earlier said a boiler at the spa had exploded.

"I heard a sudden huge boom. When I jumped outside, I saw a plume of smoke," said Miki Yamagishi, a clerk at a nearby convenience store.

After the flames had been extinguished, fire officials swarmed the blasted shell of the building, which was destroyed except for the mangled iron frame. Debris was blown into the street and hit neighboring apartment houses, shattering windows.

Shiespa is a women's-only spa in central Tokyo with a natural hot spring, an open-air bath, a sauna and salon, according to its Web site.