Fire investigators say a blaze at a south Edmonton apartment building that sent four to hospital early Sunday morning may have been deliberately set.

Investigators say there are circumstances that have led them to believe the fire is very suspicious in nature.

Emergency crews were called to a the four-storey walk-up apartment at 85 Avenue and 99 Street around 3 a.m. Sunday where a blaze had broken out in a second-floor suite.

Officials say a woman was sent to hospital with burns and three men were sent to hospital to be treated for smoke inhalation.

Residents and witnesses described the scene of the fire as chaotic.

"I woke up to people screaming, 'get out of the apartment,'" said resident Chelsea Blades.

"Everyone was screaming for people to get out. I went to open up the door and smoke was coming through the cracks, so I jumped my balcony.”

Fire Chief Dwayne Smith said ladders were used to evacuate residents of the building. The smoke was so thick that many residents were trapped on their balconies.

"We had a lot of people that were hanging outside of the balconies," Smith said.

“The majority of the people were out of the building when fire got on scene, except for those who were trapped on their balconies and fire crews did an excellent job in laddering the building and getting those people down.”

Officials say the blaze spread fast and heat was so intense that plastic on handrails in the building melted, and doors a far distance away from the suite of origin, also burned.

The fire was brought under control just before 4 a.m., with all occupants evacuated from the building.

The woman living in the suite where the fire started suffered burns to 25 per cent of her body.

“She came out the front door just bleeding and halfway on fire,” a witness told CTV News.

“Her hair was on fire and it was really scary.”

Fire investigators and EPS detectives have spoken to the woman who was taken to hospital.

They say she is in stable condition.

Investigators tell CTV News the woman was living alone in the apartment, however a man was in her suite and there was an altercation prior to the fire.

Witnesses say they saw a man riding away on a bike near the scene while crews worked to contain the blaze.  

Officials tell CTV News there was a group of people across the street who had spotted the fire and called 911. They immediately banged on doors and windows to help get people out. Investigators say had the group not risked their own lives, the fire could have been deadly.

The Red Cross is assisting 20 people affected by the fire.

Shelter, food and clothing will be provided for residents affected for the next 72 hours.

Investigators are working to determine the cause of the blaze.

The building received heavy smoke and water damage. Officials say the damage is extensive in the suite where the fire origniated and to common areas of the building.

Damage is estimated to be $500,000.

With files from Amanda Anderson