A police shooting on a downtown Toronto streetcar has left Sammy Yatim dead, a family grieving -- and unanswered questions.

Namely, how does an encounter between several police officers and an 18-year-old with a small knife on an empty streetcar end in a hail of bullets and and a discharge of a stun gun?

In a press release issued Monday, Ontario's Special Investigations Unit outlined the circumstances that led to the fatal Saturday night shooting of Sammy Yatim.

As the unit has begun an investigation, the details so far remain sparse. The SIU confirmed, however, that Yatim was confronted on a streetcar on Dundas St W., near Grace St shortly before midnight. Following an "interaction" with officers, Yatim sustained multiple gunshots wounds. Additionally, a "conducted energy weapon" was deployed.

According to multiple reports, one Toronto police officer has been suspended.

Witnesses told The Toronto Star that Yatim was brandishing a small knife. In the video, the suspect appears to shout at officers from inside the bus. Police repeatedly tell Yatim to drop the knife before shots ring out.

“Panic essentially sets in when you see cops with guns right in front of you,” Martin Baron, who watched events unfold, told the Globe and Mail. “You could hear them yelling, really, really loudly, ‘Drop the knife, drop the knife.’”

“It escalated very, very quickly, and then shots were fired," Baron added. "About 30 seconds after they stopped shooting they actually tasered him – you could hear the taser – and then an officer went in the back door of the streetcar and you could see them performing CPR on him."

Aside from pledging a thorough investigation of the case, Toronto Police Chief Bill Blair had little to say at an 11 a.m. news conference.

"I am very aware the public is very concerned," he told reporters. "They have a right to be concerned."

Meanwhile, a widely circulated, and graphic, video captured the encounter, spawning intense scrutiny across Canada at it approaches half a million views.

You can see an enhanced version of the video below.

WARNING: The videos contain graphic violence and language that might be disturbing to some readers.

Blair did, however, express sympathy for Yatim's father at the morning presser.

“As a father, I can only imagine the terrible grief and the need for answers,” he said. “We will do our best to ensure those answers are provided.”

The teen's uncle responded on Sunday. Writing to The Toronto Star, Mejad “Jim” Yatim asked, “Since when did it become a crime to be a teenager?"

His nephew, he added, had emigrated from Syria five years ago -- and was still adjusting to a new life in Canada. He had never, his uncle noted, shown signs of mental illness.

“And since when does a scrawny 110-pound-something teenager become a threat to a dozen or so brawny policemen, when he is isolated in an empty streetcar that they felt that they had no other choice but to use lethal force?” he wrote.

The community has already mobilized in the wake of the shooting, expressing support for the Yatim family through a Facebook page. As well, a vigil is planned for the city's Dundas Square at 5 pm Monday. Family and supporters plan to march from there to the scene of the shooting near Dundas and Grace Streets.

The SIU, which is an arm's length agency that probes incidents with police that involve death, injury or sexual assault, will focus on one subject officer, as well as 22 other officers who witnessed the encounter.

Investigators are asking anyone with video footage or who may have information regarding this case to call 416-622-1965 or 1-800-787-8529 ext. 1965.

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  • Next: Sammy Yatim Protest Photos

  • Sammy Yatim Protest, Vigil

    A crowd marches on Dundas Street in Toronto on Monday, July 29, 2013 where Sammy Yatim, 18, was shot nine times and killed by police early Saturday morning in Toronto. THE CANADIAN PRESS/Michelle Siu

  • A man screams at officers at Toronto Police 14 division as a vigil turns into a protest on Monday, July 29, 2013 for Sammy Yatim, 18, who was shot nine times and killed by police early Saturday morning in Toronto.

  • A man confronts officers at Toronto Police 14 division as a vigil turns into a protest on Monday, July 29, 2013 for Sammy Yatim, 18, who was shot nine times and killed by Police early Saturday morning. THE CANADIAN PRESS/Michelle Siu

  • Sammy Yatim Protest, Vigil

    A person protests at Toronto Police 14 division as a vigil turns into a protest on Monday, July 29, 2013 for Sammy Yatim, 18, who was shot nine times and killed by police early Saturday morning in Toronto. THE CANADIAN PRESS/Michelle Siu

  • Sammy Yatim Protest, Vigil

    A person chants "Justice for Sammy" at a vigil on Monday, July 29, 2013 at Dundas Street and Bellwoods Avenue where Sammy Yatim, 18, was shot nine times and killed by Police early Saturday in Torontomorning when he was alone with a knife on a street car. THE CANADIAN PRESS/Michelle Siu

  • A man shouts at officers at Toronto Police 14 division as a vigil turns into a protest on Monday, July 29, 2013 for Sammy Yatim, 18, who was shot nine times and killed by police early Saturday morning in Toronto.

  • Sammy Yatim Protest, Vigil

    Mother Sahar Bahadi is comforted as she sits next to sister Sarah Yatim at a vigil on Monday, July 29, 2013 at the spot where son and brother Sammy Yatim was shot nine times and killed by Police early Saturday morning in Toronto. THE CANADIAN PRESS/Michelle Siu

  • Sammy Yatim Protest, Vigil

    Mother Sahar Bahadi breaks down at a vigil on Monday, July 29, 2013 at the spot where her son Sammy Yatim, 18, was shot nine times and killed by Toronto Police early Saturday morning when he was alone with a knife on a street car. THE CANADIAN PRESS/Michelle Siu

  • Sarah Yatim breaks down in the arms of her mother Sahar Bahadi at a vigil on Monday, July 29, 2013 at the spot where her brother Sammy Yatim, 18, was shot nine times and killed by Toronto Police early Saturday morning when he was alone with a knife on a street car.

  • Sammy Yatim Protest, Vigil

    Mother Sahar Bahadi, right, is comforted by her daughter Sarah Yatim, left, as they march on Monday, July 29, 2013 to the spot where their son and brother Sammy Yatim was killed by police early Saturday morning in Toronto. THE CANADIAN PRESS/Michelle Siu

  • A shirt with Sammy Yatim's picture is worn by a supporter at a vigil on Monday, July 29, 2013 in Toronto at the spot where the boy, 18, was shot nine times and killed by Police early Saturday morning in Toronto.

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