Improper disposal of cigarettes led to the fire that killed two Winnipeg firefighters earlier this month, Manitoba's fire commissioner confirmed Thursday.

Doug Popowich said an investigation had determined that on the afternoon of Feb. 4, an 18-year-old resident and a friend smoked cigarettes in the garage of the St. Boniface home.

They butted the cigarettes in an ashtray, then dumped the ashtray into a plastic container that contained other refuse.

"The fact was that the ashtray was dumped into this receptacle that had combustible materials inside it. That's where the fire started," the commissioner said Thursday.

Popowich said the container was along one of the walls of the house. The fire smouldered for a while, but the young resident didn't notice the smoke until a few hours later, when he called 911.

"It's our belief that the fire was up into the walls and up into the floor space between the ceiling and the floor of the second floor, so that when the fire department went in there, there was a very well-involved fire," Popowich said.

No charges will be laid in the case, Popowich said.

Tragic outcome

Like this one, many of the fires he investigates are accidents, he said.  In this case, he added, it was an accident with a tragic outcome: the death of two firefighters.

Captains Harold Lessard, 55, and Tom Nichols, 57, were trapped on the second floor of the home when they were hit by a high-temperature flashover.

Four other firefighters were injured; two remain hospitalized with severe burns.

More than 7,500 people attended a memorial service in Winnipeg for the fallen firefighters on Wednesday.