Officials say the fire, which left three other family members in critical condition, was the city's worst in at least 40 years.
Witnesses said heroic efforts by Bacara Thach, the family patriarch, helped save his 14-month-old grandson Sipheng from the three-bedroom flat above his small grocery.
Fire crews were on the scene in moments and also saved the child's father, Thach's son-in-law, who arrived in Canada just three weeks ago. All three were listed in critical condition Tuesday in hospital.
Dead are Kol Yan Thach, her daughter Lily, 23, and three sons _ Gary, 14, Danny, 13, and Sunny, 12, according to tearful friends and neighbours gathered near the Mekong Grocery in Ottawa's Chinatown district.
''The identity of those victims is not official,'' cautioned Staff Sgt. Monique Ackland.
Autopsies are to be performed Wednesday, while fire officials had already begun combing through the wreckage Tuesday seeking the fire's cause.
The five deceased were all pulled from the burning building by firefighters but died at the scene, said officials.
''The firefighters went in four times, at risk of their own lives, in the face of a fully engulfed building to try and rescue the people,'' Ottawa police chief Vince Bevan said at a sombre City Hall news conference.
Family friend Vannary Pha said the elder Thach delivered his grandson to a neighbour and then attempted to re-enter the apartment.
''The father got burned trying to go back in,'' Pha said at the scene. ''They tried to help but they could not help because of all the fire.''
Across the street from the tiny store, a steady stream of hollow-eyed friends came to gaze at the carnage, some leaving photos and flowers.
''We had a party last Saturday with him,'' said Than Pa, a friend of Bacara Thach's and fellow member of a Cambodian community association. ''Now they're all dead. Something like that, I don't know what to say.''
Firefighters were less than two blocks away at another blaze when the fire erupted just before 2 a.m.
Some power in the area had been cut because of the original fire, but officials could not immediately confirm the Thach household was affected.
But Trevor Bain of the Ontario Fire Marshall's office said it appears the Thach fire started in a front bedroom of the apartment, not the store below. Some neighbours were already suggesting Tuesday morning that a candle ignited the blaze.
Fire officials used the tragedy to remind people of the critical importance of smoke detectors in giving fire victims a chance to escape.
Donny Dao, 13, last saw his friend Gary Thach two days ago when they gathered to play poker.
''I've been crying for two hours,'' said Dao, hands stuffed in pockets as he stood on the street outside the burned-out store.
The three brothers shared a bedroom, a love of jokes and volleyball and excitement about their new nephew, said Dao.
A teary-eyed Phea Meas said the Mekong Grocery attracted Cambodians from across the city by dint of the family's personality. They had been in Canada about 20 years, with only the eldest daughter Lily born overseas.
Yet the Thocks maintained strong connections with Cambodia and were known for organizing fundraising drives for its people.
''It's very hard for all the Cambodians here in Ottawa to cope with the situation right now,'' said family friend Sao Thok.
''We are shocked. We lost part of our family here. We lost our best friends.''
