Police said the fire began in a small building where lumber is cut, but did not spread to the store.
Debbie Euchner, a City of Yellowknife spokeswoman, said the firefighters were inside the building when the roof caved in on them.
``During the process of suppressing the fire, firefighters were on the roof trying to make a hole to tackle the fire which had spread through the trusses,'' she explained. ``During this procedure the roof collapsed upon two firefighters who were inside the structure.''
She said they were taken to hospital, but doctors couldn't save firefighter Kevin Olson, 24, who had just joined the department 11 days earlier.
Olson's 18-year-old widow, Erica Olson, said her husband always said he would probably die in a fire.
``He was on his eighth shift, his first night shift, there were a lot of firefighters at the scene, why did he have to be one of the two that went in?'' she asked tearfully.
``He's a rookie _ he was so young, it wasn't his time. It's just not fair.''
The couple had gotten married in Calgary on Jan. 28. They had moved to Yellowknife just two weeks ago.
``I still think that maybe it's just a big mistake, that he's still alive and that he's going to call me and tell me that he loves me,'' she said. ``But I am really proud of him and I know I will see him again someday.''
Lieut. Cyril Fyfe, 41, an 18-year veteran with the city fire department, was in serious condition, Euchner said. She didn't know the nature of his injuries.
No other firefighters were injured in the blaze. The cause has yet to be determined, but a heater is suspected.
Bryan Rendell, 42, who owns a glass repair shop across the street from the hardware store, said he saw three firefighters climb up onto the roof with a chainsaw, crowbar and a snow shovel just before the roof collapsed.
He said they cut a hole with the chainsaw and had been there for about five minutes, working on a second hole, when the roof fell in.
``Basically all we saw was . . . hard hats flying in the air and this thing fell out from under them,'' he said.
Rendell said smoke was pouring from both ends of the shed and there were police and four fire trucks on the scene.
He speculated the roof may have collapsed under the combined weight of a large amount of snow and the firefighters.
``When I looked over there, and I saw them over there, my first instinct was, well, there's a lot of weight on that roof, so it probably wasn't the safest place to be.''
Euchner said Olson's death is the first in the line of duty in the Yellowknife fire department's history.
``Firefighters are grieving,'' she said. ``The city of Yellowknife is very sad. The mood is very sombre and our hearts and prayers go out to the families.''
Mayor Gordon Van Tighem expressed condolence's to Olson's family on behalf of city council members and staff.
Yellowknife has a volunteer component to its fire department, but the two men injured in the accident were both full-time employees, Euchner said.
Fyfe, who has five children, worked as a volunteer for two years before he became a full-time firefighter.
Building owner Chuck Corothers told Global TV there were two electrical heaters in the roof of the shed. He said damage seemed to be concentrated in the shed's ceiling.
``It's probably one of the heaters that might have caused it,'' he said.
Fire department officials were not immediately available to comment on the blaze or how it may have begun.
Rendell said the shed was used to cut plywood, lumber and mouldings for customers.
He expressed grief for the fallen firefighters.
``It's pretty sad, especially in a small-knit community like Yellowknife where everybody knows everybody,'' he said.
Fire crews were called in about 7 a.m. when a municipal enforcement officer spotted smoke after stopping a speeder on Yellowknife's Old Airport Road.
Worker's Compensation Board officials, the fire marshall's office and the Yellowknife fire department are all investigating the fire.
