Police say a 63-year-old man remains in hospital with life-threatening injuries from the blast and ensuing fire.
Authorities had originally reported two individuals were in critical condition.
A firefighter also had minor injuries.
Firefighters say a preliminary investigation indicates the explosion occurred near a ground-floor staircase.
Roughly 100 firefighters were on the scene shortly after the 2 p.m. explosion, along with police and paramedics.
Police cordoned off several streets while experts examined the building to ensure it was stable.
Spokeswoman Lynn Duncan said the investigation was transferred to the police arson squad to determine what exactly went wrong.
"The initial cause would be a propane tank explosion, causing a fire that was quickly controlled by the firemen," she said.