A three-alarm house fire Wednesday afternoon in a densely packed Chelsea neighborhood that displaced up to a dozen residents may have been started by a cigarette butt left on a back second-floor porch, a resident said.

“I ran downstairs to the second floor, and the guy is pouring water on his back porch,” said Awilda Morales, 41, a mother of three who lived in the building on Congress Avenue. Her children were at school. “I asked what happened, and he said, ‘It’s on fire, and I can’t turn it off.’