When the giggles at the rear of the streetcar were swallowed by a piercing shriek, Aaron Li-Hill looked back and saw a flash of light.

Three girls darted past him toward the front of the moving streetcar, fleeing whatever it was that gave them such a fright, and soon all of the 15 or so passengers were at the front, too – all except Mr. Li-hill and one young man. The flash, Mr. Li-Hill had realized, was the blade of the man's knife.

Mr. Li-Hill, a 27-year-old artist, was the last passenger to interact with 18-year-old Sammy Yatim before the teen was shot and tasered by Toronto Police early Saturday. His account is among the new details that emerged Tuesday about the shooting that has riveted the city, spurring debate among those trying to make sense of why the young man, who had got lost trying to meet up with a friend, was shot. Protests have ensued, a memorial has been built and investigations are under way.

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Having just bicycled to pick up his girlfriend from a bar so the pair could travel together on the Dundas streetcar, Mr. Li-Hill made the snap decision to use his bike as a protective shield and buy other passengers time to run away.

"He stuck his arm out and the knife was sticking straight up in front of me," Mr. Li-Hill said Tuesday. Mr. Yatim screamed at passengers not to get off the streetcar.

"I was backing up holding my bike saying, 'Please just let us off the streetcar. Just let us off the streetcar,'" Mr. Li-Hill said.

Cellphone video of the teen's shooting death has been circulating for days – a total of nine shots rang out, it shows – but new video from a nearby convenience store emerged late Monday, showing Mr. Yatim lying on the streetcar floor before the final six shots were fired.

On Tuesday, the officer involved in the deadly shooting was identified as Constable James Forcillo. His name can be heard in a new cellphone video posted to YouTube when, shortly after Mr. Yatim is shot, an investigator shouts, "Where's Forcillo? Find me Forcillo."

Also Tuesday, Mayor Rob Ford commented on the shooting for the first time, calling it "unfortunate" and offering his condolences to the teen's family.

Mr. Yatim's family, for their part, issued a statement in the afternoon expressing their heartbreak, faith in the Canadian justice system and gratitude for the public's support.

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"We are living a nightmare we can't seem to wake up from," the statement said. "We want to be clear that we do not hold any ill will against the thousands of police officers who work to protect us each day. This is a tragedy for all involved."

In Mr. Yatim's final minutes, according to Mr. Li-Hill, the teen then changed direction and began yelling at the passengers and driver to get off the streetcar. But Mr. Yatim never angled the knife toward him and never tried to thrust the weapon through the bike frame as Mr. Li-Hill stumbled backward toward the front of the streetcar, Mr. Li-Hill said.

"We walked, locked in this moment," Mr. Li-Hill said. "I was staring at his eyes and his hands the whole time … His eyes were extremely wide open, his jaw was really clenched – just sort of a crazed look. Very intense. But my initial impression was, 'What is this young kid doing? What is going on in his head right now?'"

Mr. Li-Hill's girlfriend managed to get to the front of the streetcar and started yelling, "Aaron! Aaron! Get out of there!" Mr. Li-Hill said he planned to throw the bike at Mr. Yatim if he attacked, but that moment never came. Instead, Mr. Li-Hill manoeuvred the bike around the corner and out the front door, joining his girlfriend and the rest of the passengers on the street.

But the driver was still on board.

"There was an odd moment where I looked at the streetcar driver and said, 'Are you okay? Do you want me to call the police?' " Mr. Li-Hill said. "And just then, Sammy looked over at the streetcar driver and I had this thought of, 'Oh crap, this could be really bad – [Sammy] hadn't realized [the driver] was there.' But Sammy just looked at him and then looked back out."

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The driver got off and emerged unscathed, and Mr. Li-Hill and his girlfriend left the scene on foot before police arrived. They were still nearby, though, when the shots rang out and Mr. Li-Hill thought, "Wow, the situation must have really escalated." He has since been interviewed by the Special Investigations Unit, which is probing the shooting.

Tuesday night, people walking along Dundas Street stopped at the spot where Mr. Yatim was killed, reading the signs and messages posted to a telephone pole that has become a memorial.

"They shot him nine times, he was just a kid. I mean what's more powerful, a knife or a gun?" said Paul Girllo, 42, who lives at Bloor Street and Lansdowne Avenue. "Why wouldn't they use a taser gun? I just find it appalling and disgusting. There was nobody with the kid anyway. He was on the streetcar by himself. I don't see how much more de-escalation there needed to be."

Keely Buckley, 23, also called the incident "disgusting," as she looked at the memorial with a friend. "The kid wasn't doing anything, he had a knife, and they had 12 people with guns. Who does that?"

With reports from Adrian Morrow, Vidya Kauri, Patrick White, Daniel Bitonti and Cynthia McQueen

Editor's note: This article has been updated with information that did not appear in the original print version or in an earlier online version of it. Sammy Yatim told people to get off the streetcar, according to witness Aaron Li-Hill, after initially telling them to remain on it.