EDMONTON -- Investigators know the flames that overtook a seniors' home in St. Albert, causing 110 residents to flee, started on a ground-floor patio but not how.

In an update on Friday, St. Albert fire safety codes officers and the fire department revealed they had been able to determine the path of the blaze, but there was too little evidence to conclude how it started.

"The fire was initially undetected on the patio, which allowed it to grow. It spread from patio furniture to the underside of the deck structure above," read a statement from officials.

"From there, it travelled up the vinyl siding into the attic of the building and eventually the roof."

Firefighters were called to Citadel Mews just after 7 p.m. on May 6 and called for backup from Edmonton a short while later with flames quickly spreading across the building. Officials on scene that night called it "overwhelming."

According to investigators, the fire was "extremely difficult" to access once it reached the attic, and high winds didn't help.

Dozens of residents were evacuated from the facility and a sister building next door, including 40 who were living in extended medical care suites and needed to be transferred to nearby hospitals. One resident was found still in her room hours after the blaze initially broke out, and three were taken to hospital for smoke inhalation and minor lacerations.

A damage estimate has not been made. Officials say most of the damage occurred to the top and ground floors where most of the flames were, but that there was smoke and water damage throughout Citadel Mews.