Federal investigators have determined that the large fire that destroyed the Winthrop post office last week was accidental and likely the result of a mechanical or electrical malfunction, a spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives said Tuesday.

An inspector from the federal agency, who is based in Maine, came to that conclusion after making two trips to the burned out post office, said the spokesman, Christopher Arone.

While the inspector did not determine the exact cause of the blaze, which started in the building’s attic, his findings were enough for the federal agency to determine there was no foul play, according to Arone.

“Bottom line, it was not intentionally set,” he said.

The U.S. Postal Service plans to tear down the post office after the fire last week, which gutted the interior of the building and collapsed its metal roof, a spokesman for the Postal Service, Steve Doherty, said on Monday.

“They’re going to need to demolish what’s left of the building,” Doherty said. “The plan right now is just to rebuild on the same location. That’s what (the Postal Service is) moving towards.”

Doherty wasn’t able to offer a timeline for that work.

The post office had operated at 112 Main St. for the last seven years after replacing an older facility that was difficult for handicapped patrons to enter. Firefighters responded to the fire at 8:54 a.m. shortly before it opened for business, but the post office lobby was open to the public.

After employees noticed smoke coming from a ceiling tile and heard a crackling sound, the building was evacuated, Doherty said last week. No one was injured during the response.

For the time being, Winthrop residents and businesses that had post office boxes will have to pick up their mail at the Manchester post office at 31 Readfield Road, Doherty said. Delivery of mail to Winthrop addresses will continue out of the Augusta post office, but items that require a signature and can’t be delivered will be left in the Manchester location.

Charles Eichacker — 621-5642

[email protected]

Twitter: @ceichacker