The incident took place around 9:30 a.m. at the fifth-floor offices of Ceridian, which provides payroll and "human capital management" services.
"He was being fired and I guess then he proceeded to get involved in stabbing some of his bosses and some other employees," Det. Daniel Darnbrough told The Canadian Press.
Police charged Chuang Li, 47, of Mississauga, Ont., with three counts of attempted murder, four counts of aggravated assault and four counts of assault with a weapon. He was set to appear in court on Thursday.
Ceridian would only describe the man as a "former employee."
"This is a very difficult time. The entire Ceridian family is shocked and deeply saddened by this incident," the company said in a statement. "All four employees are being treated by medical experts at area hospitals."
Police said the suspect — who was arrested at the scene — used a "sharp instrument...possibly a knife before those around him were able to hold him down.
"I believe the employees eventually subdued him until the police got here," Darnbrough said.
Police said a 32-year-old woman and three men between the ages of 35 and 45 were taken to hospital. The woman's injuries were described as life-threatening.
The suspect was not injured in the incident. Footage from outside the office building showed a bespectacled man wearing a striped blue office shirt and dark jeans being led into a police cruiser.
"We still have a lot of forensic evidence to gather here," Darnbrough said."It's an ongoing investigation."
Steve Paraskevopoulos, who worked on the fourth floor of the building, recalls hearing a commotion coming from above.
"It was kind of a stampede kind of sound," he said. "It was an odd thing, we have thick slab concrete floors, you never hear a thing from somebody above you unless there's construction."
Shortly after, Paraskevopoulos and a few co-workers headed down to the ground floor to get coffee and ran into one of the building's security guards who told them "something serious" was going on.
Paraskevopoulos then saw a number of police cruisers, ambulances and fire trucks outside the building, and said one of his colleagues also noticed officers head into the building with their guns drawn.
It was at that point that Paraskevopoulos saw the victims, who were smeared with noticeable traces of blood.
"There was one guy who looked like he took a stab across the head, it was all bandaged up," he recounted.
"There was another gentleman who kinda was sitting on the side beside the EMS truck...and I definitely saw two people come out on a stretcher that looked to be a lot more serious. They rushed into the back of the ambulances."
Tony Bitonti, who works with the Ontario Lottery and Gaming Corp in the building next door, was able to get a closer look at two of the men who appeared to be less seriously injured.
"One had a bandage around his head and also around his jaw," he said. "Another young guy had an undershirt on, like a white T-shirt, and it was ripped, the left hand sleeve was gone."
Bitonti said the two men were able to walk to a waiting ambulance.
The entire incident appeared to have rattled many working in the office complex.
"It was more stunned disbelief than anything," Bitonti said of how the people around him were reacting to the stabbing. "It's fairly quiet, fairly tranquil around here and when something like this happens, it's weird."
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