They say the unarmed officer was arresting a man for trespassing when an accomplice pulled a hand gun and threatened the officer.
Both the trespasser and the accomplice, believed to be 23-year-old Abdullahi Mohamoud of Mississauga, Ont., fled.
Mohamoud is wanted on a Canada-wide warrant after he was charged with killing a gas station attendant in Ontario.
"He has an extensive criminal history, most important of which is a Canada-wide warrant for criminal negligence causing death from the Peel Region in Ontario where it's alleged that he dragged a gas station attendant to his death," said Staff Sergeant Malcolm Allan.
Police believe Mohamoud filled his car with about $75 dollars worth of gas in May 2011, then left without paying.
Mohamoud then ran down and dragged to death the 62-year-old attendant who tried to stop him, said police.
"He's a dangerous individual," Allan said. "He's considered armed and dangerous and anybody who obviously sees him or locates him is to not approach him or take him into custody themselves but to call us."
Mohamoud is described as 6’2" and weighing 160 lbs. He was last seen wearing a white shirt over a dark hoodie, blue jeans and black shoes.
The trespasser was arrested a few hours later by downtown division beat members.
Mohamoud is also wanted for aggravated sexual assault for an incident in 2008 in Edmonton.