By Steve Tawa

WEST CHESTER, Pa. (CBS) — Federal investigators now say that they have pinned down where the deadly West Chester assisted living complex fire began, but not how it started, in mid-November.

READ MORE: 21-Year-Old Eddie Rodriguez Killed In Road Rage Shooting On Roosevelt Boulevard, Police Say

The Bureau of Alcohol, Tobacco, and Firearms agents, including forensic experts and engineers, believe the starting point of the massive blaze was “the exterior of the building, in the area of a covered patio” at Barclay Friends Senior Living Community. But there’s no word yet on how it began.

A spokeswoman for the ATF, Charlene Hennessy, says there’s “no evidence to suggest that it was an arson, and it appears to be accidental,” but investigators are still doing their work. She anticipates an ATF briefing will be held next week.

READ MORE: WATCH LIVE: Delaware County DA Jack Stollsteimer To Announce Charges In Shooting Outside Upper Darby SEPTA Terminal

As to whether the fire suppression system was operating properly when the five-alarm fire broke out and quickly spread, and whether the sprinklers did not kick in, Hennessy says investigators were still doing additional testing.

Most of the investigators’ work centers on determining exactly what touched it off, as well as tracing its path through the sprawling complex.

Three-and-a-half days after the fire, a grim Chester County District Attorney Tom Hogan disclosed four residents died.

MORE NEWS: 2 Men Found Dead Outside Home In Tacony Neighborhood, Philadelphia Police Said

He added, because of the speed of the blaze, fanned by a wind that he described as “whistling through the night,” and the condition of residents, 40 to 50 people could have been lost.