TORONTO — The lawyer for a Toronto police officer on trial for killing a teenager on an empty streetcar ripped into the credibility of a use-of-force expert on Wednesday, suggesting the key prosecution witness had a problematic "lack of practical experience."

Peter Brauti's barbs were hurled at Robert Warshaw, who testified this week at Const. James Forcillo's trial.

"I'm going to suggest you're way, way in over your head," Brauti said during a spirited cross-examination session. "I'm going to suggest your lack of practical experience is a problem in this case."

Warshaw — a former U.S. police chief who has helped several forces reform their operations — has testified that Forcillo had multiple alternatives to lethal force before he shot Sammy Yatim in July 2013, but didn't use them.

Forcillo has pleaded not guilty to second-degree murder and attempted murder in the death of the 18-year-old.

The jury has heard that Yatim had taken the drug ecstasy at some point before boarding a streetcar on which he eventually pulled out a small knife, causing panicked passengers to rush off.

Surveillance videos have shown Forcillo arriving at the scene, yelling at Yatim to "drop the knife" in a confrontation that lasts 50 seconds, after which the officer fires nine bullets in two volleys.

Warshaw has testified that Yatim appeared to pose "zero to negligible risk" and has suggested Forcillo could have taken more time to deal with the situation in a way that wouldn't have resulted in the teen's death.

Brauti suggested, however, that Warshaw was engaging in "Monday morning quarterbacking."

"We don't want people at the water cooler, unqualified people, giving opinions about what could have been done, what should have been done," Brauti said.

"You haven't been involved yourself in having to make a use-of-force decision in over a decade or longer."

Brauti noted that Warshaw received his own basic police use-of-force training decades ago and isn't a certified use-of-force instructor.

He also said that while Warshaw reviewed the training Forcillo received at the Toronto Police Service, he did not review the basic instruction the officer received at the Ontario Police College.

"It's very important we understand the training officer Forcillo received," Brauti said before noting that Forcillo pointing his gun at Yatim and commanding the teen to "drop the knife" were accepted de-escalation efforts.

Warshaw agreed, but said he would suggest "a variety of different approaches" to defuse the situation.

Brauti, however, went on to pick apart Warshaw's recommended actions, suggesting they would not have worked for Forcillo when he was faced with a knife-wielding Yatim.

"Your suggestion to officer Forcillo ... is he should have yelled do you need a drink of water? Should we call someone?" Brauti said.

Warshaw countered that Brauti was making light of his suggestions.

Brauti also criticized Warshaw's suggestions that police could have thrown something at Yatim, like a baseball, a paint can or even an officer's expandable baton, to distract him and avoid a lethal outcome.

"You know fundamentally the first principle is you never give up a use-of force option," Brauti said of the baton idea in particular. "It's one of the worst ideas anybody could come up with."

Brauti further derided Warshaw's suggestion that police could have used a switch on the outside of the streetcar to close the vehicle's doors and contain Yatim, saying police didn't know if there was anyone else stuck on the vehicle.

"If Mr. Yatim stabs that person and they can't get out, you know what happens, right? They say to the officer 'why did you close the doors,'" Brauti said.

"This is the point, you don't have the tactical knowledge to be speaking about these things in a court of law, do you?"

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Warshaw countered that any officer who believed there might be someone else might be on the streetcar with Yatim wouldn't have fired.

"If in fact there is a hostage or an innocent party you would delay at all costs," he said. "If that officer really believed that there was someone else in that streetcar and discharged nine rounds of ammunition ... that officer's behaviour in those circumstances would be reckless."

Crown prosecutors have said they plan to prove that Forcillo's actions during the incident weren't necessary or reasonable. Forcillo's lawyer has said his client's actions were justified and carried out in self-defence.