Peel Regional Police say they are treating the fire at a Mississauga office building as an arson investigation and they have released images of a possible suspect.

"It is alleged that at approximately 4:30 p.m. the suspect entered the building and set the fire on the third floor," police said Thursday in a news release about the fire at the Erindale Corporate Centre building at 1270 Central Parkway.

"The suspect is described as a white male, 5’7” tall, approximately 200 lbs, with a heavy build. At the time, he was wearing orange coveralls, a black t-shirt, a black hat and dark shoes."

A 911 call was placed at 4:37 p.m. on Wednesday. An office worker for a Conservative MP tried to extinguish the fire that may have started outside his office's doors.

"I went to see how bad the fire was, and the flames actually started coming in the office," Dan Duncan, assistant to Bob Dechert (Mississauga-Erindale), told reporters on Thursday.

"It spread very quickly. I closed the door again, went into the office, grabbed the fire extinguisher that we keep there and was able to push back the flames a little bit -- to be completely honest, not a whole lot."

His fire extinguisher ran out. Duncan said he closed the door again, but was able to make it out of the third-floor office and down to the second floor where he pulled a fire alarm.

"I grabbed a second fire extinguisher on that floor, and tried to go back into the stairwell and see if I could do any more damage to the fire. I quickly realized that wasn't going to be at all effective. The smoke in the stairwell was … a large amount at that time."

The fire broke out on the third floor. Thick smoke rose up to the fourth, fifth, and sixth floors, leaving the workers unable to use the stairwells as they waited for firefighters to rescue them.

Only one person was taken away by ambulance for treatment of smoke inhalation, but 250 people were put at risk.

The building is closed Thursday, and police say there is no indication as to when it will reopen. Damage is estimated at $5 million.

"All MPs hear complaints from time to time and get angry emails, but there were no specific threats of violence against my office or any of my staff," Dechert said Thursday.

"It could have been targeted against my office or one of the other offices in the building. But whoever did this is obviously a very disturbed individual and I hope they get the assistance they need soon."

"Anyone with information on this matter is asked to contact investigators at the 11 Division Criminal Investigations Bureau, at (905) 453-2121, ext. 1133," police said.

People can also call Crime Stoppers anonymously at 1-800-222-TIPS (8477).

With a report from CTV Toronto's Jim Junkin