Investigators are trying to figure out what caused a fire in a downtown Edmonton highrise that forced more than 100 people from their homes and sent four to hospital Thursday night.

Crews at the scene said the fire started in a unit on the sixth floor of the 20-storey building.

The first six floors of the building are dedicated to assisted living, and are home mainly to seniors.

The call for help came just after 11 p.m. Firefighters evacuated 80 to 100 people from the fifth, sixth and seventh floors of the building.

Several more from higher floors left when they heard the alarm and saw the fire trucks.

Four people went to hospital.

Emergency crews closed off Jasper Ave. and an Edmonton transit bus arrived to provide shelter. Dozens crowded into a dining hall on the main floor of the highrise.

Firefighters had the fire out and turned the scene over to fire investigators by about 2 a.m. on Friday.