The fire that destroyed the beloved Frager’s Hardware store this month on Capitol Hill probably ignited after an employee dropped a cigarette butt into a bucket in an alley, said two law enforcement officials with direct knowledge of the investigation.

Officials believe the fire was accidental, and they say they’ve interviewed a 23-year-old employee who was seen on surveillance video taking a smoking break 30 to 40 minutes before the fire was first reported about 6:30 p.m. June 5.

The alley, which sits between the Frager’s complex and a neighboring catering business, was used to store wooden beams, bags of cement and a host of other supplies for the paint and hardware shops. A jumble of wires and extension cords initially piqued the interest of investigators, but the probe soon narrowed to a two-gallon bucket used by employees on cigarette breaks, the two sources said. That plastic bucket has now been identified as the fire’s point of origin.

Officials said it was unusual but not unheard of for a cigarette to cause such a large fire. The conditions that day created what one investigator called a “perfect storm” — a slow, smoldering fire in the bucket fueled by a commercial fan blowing air through the narrow alley.

The same source said that mild wind gusts added more oxygen to the growing fire.

Officials said the bucket melted into a pool of flames that ignited nearby plywood lining a metal fence. Flames leaped to a plastic roof covering half the alley, which melted and dripped onto piles of wood, a saw table and other debris, causing the thick black smoke that billowed over the city.

It took more than 200 firefighters more than five hours to bring the blaze near Eastern Market under control, and they were still dousing the debris with water the following morning.

Frager’s Hardware had been an institution on Pennsylvania Avenue Southeast for 93 years, and it had grown to include a paint, hardware, flower and equipment rental shop, all located in adjoining or neighboring buildings. Customers and Capitol Hill residents were devastated by the fire, lamenting the loss of a store that seemed to have everything imaginable crammed into its packed shelves and narrow aisles.

Mayor Vincent C. Gray helped the owners open in a temporary stall in Eastern Market over the weekend after the fire, and residents showed their solidarity by wearing blue Frager’s T-shirts and contributing to fundraisers to help out-of-work employees.

The store’s owner, John Weintraub, said he has not been told the probable cause of the fire, despite repeated attempts to get information from officials. He said authorities told him they could not comment until the final report is released, possibly next week.

“Nobody is ever going to smoke in any store that I have again,” he said, after learning that a cigarette butt was the likely cause.

He said he knew workers were smoking in the alley but that he had tried not to speculate about the cause. He said his employees constantly walked through the alley to reach other buildings, retrieve supplies or find products for customers.

“I’d be curious to know how the investigators are pinning this down,” he said. “People are in that area all the time. It surprises me that something could smolder for a half an hour and nobody noticed.”

Weintraub said he was in his store when the fire broke out and went to the alley to try to put out the flames with a fire extinguisher.

“I opened the door, and it was just black smoke,” Weintraub said. “I didn’t see any flames. I ran around to the other side of the alley and saw the flames.”

He wants to rebuild but is waiting for the final investigative report and information from his insurance company.

A spokesman for the fire department, Lon Walls, declined to comment, citing the ongoing investigation.

Because of the fire’s size and complexity, the investigation was led by a task force made up of District firefighters, D.C. police and members of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.