A Mississauga teen saw his manslaughter trial begin in a Brampton courtroom today, one that has its genesis in a high school rugby game two years ago.

Witness Craig Inward testified Monday that the accused hit Castillo with a "spear tackle," driving him into the ground.

The victim, 15-year-old Manny Castillo of the Lorne Park Spartans, suffered a severe head injury. He died several days later after being taken off life support.

Inward said the accused -- who cannot be named due to provisions in the Youth Criminal Justice Act -- went out of control after being called for an infraction against Castillo in the May 7, 2007 match, "stamping his feet and swearing at the referee."

It soon became clear Castillo was seriously injured, he said.

Inward said the ball was at least 20 metres away when the incident happened, with no apparent reason behind the rough play.

A rugby coach himself, Inward said players can only tackle or contact opponents who are either in pursuit of the ball or have it in possession.

The 18-year-old accused, then a student at Erindale Secondary School, had originally been charged with aggravated assault. Police upgraded that to manslaughter, which means they believe the accused unlawfully caused the victim's death but didn't intend it to happen.

"Had he walked up to Manny Castillo on the street and slammed him into the ground and he died, (the accused) would be guilty of manslaughter," Crown attorney John Raftery said in his opening remarks.

"There is nothing magical because it happened on a rugby pitch."

Coroners conducted a post-mortem exam on Castillo, but there's no record of a cause of death being made public.

"It's probably one of those things that's evidentiary in nature, so it'll have to wait for the trial to come out," Const. Wayne Patterson, a Peel police spokesperson, told ctvtoronto.ca on Monday.

After Castillo's death, Manuel Castillo, the boy's father, described what happened as an accident.

"(If) somebody took a gun and shot my son, it's a different story, tried to assault (him), it's a different story, but an accident in a game? I don't think that boy is trying to kill somebody," he told CTV Toronto in a May 13, 2007 interview.

Manny's organs were donated through the Trillium Gift of Life Network.

The accused has been playing hockey in the Ontario Hockey League.

With a report from CTV Toronto's John Musselman and files from The Canadian Press