Police released this image of a man they say set a fire on the third floor of a building at 1270 Central Parkway W. in Mississauga, Ont. (Peel Regional Police)

A fire that consumed a Conservative MP's constituency office in Mississauga, Ont., was arson, investigators confirmed Thursday.

The three-alarm fire started on the third floor of the building at 1270 Central Parkway W. at about 4:35 p.m. Wednesday.

Peel regional police alleged Thursday afternoon that a man entered the building at about 4:30 p.m. and set the fire on the third floor, which houses the constituency office of Mississauga-Erindale MP Bob Dechert.

Police described the man as white, five feet seven inches tall and about 200 pounds. He wore orange coveralls, a black T-shirt, a black hat and dark shoes.

Dechert's office was heavily damaged in the blaze. An insurance company also had an office on the floor, according to Dechert.

When asked by reporters earlier Thursday about any threatening correspondence, he said, "There were no specific threats against me or my office in recent days.

Firefighters prepare to enter a room next to MP Bob Dechert's office in a Mississauga building. (CBC)

"Nobody has said: 'I'm going to do harm to you or your staff.'"

He said people who contact an MP's office usually call because they're unhappy with something, so it's not unusual for his staff to deal with upset people.

Estimated $5M in damage

The fire was limited to the third floor, although people on the fourth, fifth and sixth floors had to contend with heavy smoke, police said. Some barricaded themselves in offices in an attempt to stave off the fumes.

Mississauga firefighters arrived on the scene in short order and put out the blaze on the third floor. They helped an estimated 250 people get out of the building safely, although a few people suffered minor injuries.

Damage to the building is estimated at $5 million, police said.

Dechert said he is relieved everyone got out safely. He credited a member of his staff, Daniel Duncan, with helping to shepherd people from the third floor.

Duncan heard a loud noise in the hallway outside his office and saw an orange glow coming through the keyhole door. He cleared a path to a stairwell with a fire extinguisher and helped some of the floor's occupants get out, Dechert said.

The building was closed Thursday while authorities investigated.

Police are appealing to the public to contact them with any information.