Charlottetown's fire inspector says the cause of a fire that destroyed an apartment complex in the city's downtown has been linked to garden mulch outside the building.

Winston Bryan says Wednesday's pre-dawn fire originated in the mulch at the back left corner of the building, although exactly what caused it to smoulder and then ignite hasn't been determined.

More than 50 people, mostly seniors, were able to safely get out of the building on 10 Harley Street.

Bryan says witness information and security video footage has helped with the investigation.

He says people smelt and first saw smoke in the area around 8:30 p.m. Tuesday, and the video shows smoke in the early hours Wednesday before the fire ignited and leapt up the back of the building with the help of dry timber decking on the balconies.

Bryan says a propane tank explosion also helped accelerate the blaze.

"That was a contributing factor to the fire growth, and quick fire growth," said Bryan.

He said the three-storey building had no sprinklers, and wasn't required to according to building limits.