The fire that left a dozen people homeless in Waterville on Monday began in an apartment in the rear of the building, investigators said, but additional work will be required to determine the cause.

Sergeant Ken Grimes of the Office of State Fire Marshal said Thursday that the Paris Street fire began in a second floor apartment. “I believe it was in the bedroom,” he said.

The blaze at 15 Paris St. broke out at 10:12 a.m. on Monday. By nightfall on Tuesday, the building, which had been home to nine adults and three children, was being torn down.

Grimes said it was not clear where in the bedroom the fire started. Because the building was razed within hours of the fire being put out for public safety, investigators have had to rely on what witnesses say they saw and heard in figuring out what happened in the fire.

“The investigation is going to be all witness driven. Witnesses placed the fire in the bedroom of that apartment,” he said.