Fire investigators are still working on the cause, but they say it appears Sunday's massive blaze in Nepean began outside, on a second-floor balcony of one of the units.
Ottawa Fire Service spokesperson Danielle Cardinal says investigators are leaning towards an accidental cause, so arson investigators are not being called in.
As many as 70 people wre made homeless by the four-alarm fire on Northview Road.
The blaze destroyed or caused heavy damage to 32 of the units.
Ottawa Fire Chief Gerry Pingitore said it took about 80 firefighters to battle the stubborn fire in an attack that was made more difficult by frigid temperatures and high winds.
One witness said he saw smoke standing from the Walmart Supercentre in Laurentian Place, a little more than a kilometre away.
No injuries are reported and damage is pegged at $3.5 million.