WATERTOWN — Rose Reggio was praying the rosary in the bedroom of her Watertown apartment on Monday afternoon when she heard the explosion. The house began to fill with smoke, and suddenly there were firefighters helping her down the back stairs.

“I heard a big bang, there was all this commotion. . . . I didn’t know what was on fire,” said the 87-year-old. “You pray, you pray to God.”