Two weeks after a downtown fire gutted part of one building and closed three businesses, fire marshals have conceded that they may never know what caused it.

Barring some unforeseen evidence in the case of the fire at 149 W. Washington St., Suffolk spokeswoman Debbie George said Thursday, the cause will remain “undetermined” in the fire marshal’s report.

The blaze started in an apartment above the Cherry Gift Shop. That apartment and the roof of the building were destroyed. The gift shop downstairs, along with two businesses in the adjoining building, Red Thread Studio and Dave’s Fitness Coaching, suffered smoke and water damage.

All three businesses remain shuttered while cleanup efforts continue and their owners decide whether to reopen.

A benefit fundraiser is planned for Sept. 12 at the Suffolk Museum for Red Thread Studio. Organizers plan a silent auction, and are asking for donations of products, gifts, food and cash to help out.

Call Nancy Kinzinger at 514-7284 for more information.

Red Thread Studios, which specialized in yarn and fabric art pieces, was especially hard hit, because most of its stock could not be salvaged after the smoke damage, owners Angelia and Steve Armstrong said after the fire.

Adding further stress to already-jangled nerves, a smoke detector alarmed in the Red Thread building at about 11 a.m. on Thursday, resulting in another massive response by Suffolk Fire and Rescue.

Firefighters arrived on scene in full turnout gear. Pumper trucks, ladder trucks and other equipment blocked the road in front of the store all the way to Main Street.

Angelia Armstrong stood on the sidewalk awaiting word about the alarm as firefighters with thermal imaging equipment scanned the buildings. No danger was discovered, but the response had been reminiscent of what downtowners had seen two weeks earlier.

The adjacent building, which housed the Cherry Gift Shop, had last been inspected by fire marshals in 2002, according to George, who said officials had told her such inspections are not mandatory for most Suffolk businesses.

During that voluntary 2002 inspection, she said, inspectors were on the lookout for “life safety issues” — things like improperly marked exits, improper storage of hazardous materials and non-functional safety devices.

Fire marshals visit various parts of the city on a rotating basis, asking business owners for access to their properties in order to complete the inspections, George said.

When they visited the W. Washington Street area downtown earlier this year, nobody answered a knock at the door of 149 W. Washington St., so the Cherry Gift Shop and its apartment upstairs — which evidence indicated may have been occupied at the time of the fire — missed getting an inspection.