A cleaning agent used on an old oil barrel is the likely cause of an explosion that killed high school student Eric Leighton three years ago, an inquest heard Monday.

Leighton, 18, died and several other classmates were injured on May 26, 2011 when a barrel he was cutting in shop class at Mother Teresa Catholic High School exploded.

At an inquest into Leighton's death, Ottawa Fire Service investigator William Hay said it was a preventable incident.

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Eric Leighton was less than one month away from graduating high school when he died in a school explosion. (Photo courtesy of Leighton family)

"It was clear to us right at the outset that there existed legislation just to prevent this type of incident from happening, cutting into an enclosed barrel is against the fire code and if you do utilize any form of solvent or agent inside of a vessel such as a barrel, you are expected to test if there is any of the volatility left inside," said Hay.

Earlier the inquest heard from Dr. Christopher Milroy, who conducted the autopsy on Leighton.

He said Leighton died of severe head trauma and would have become "immediately unconscious" after the explosion. Milroy also said Leighton could not have survived the injuries he suffered.

​Leighton's mother Sheri said it was a small comfort to know her son "didn't suffer."

Photos of explosion aftermath shown

Eric Leighton's parents Sheri and Patrick were in attendance and said the evidence has been important.

"We hadn't seen any of the photos before, and the extent... it was unbelievable," said Patrick Leighton.

"So it's not surprising with the extent of the blast... I'm surprised only one person had a casualty," said Sheri Leighton.

The first to testify Monday was Ottawa police officer Perry Biddiscombe, who brought photos from the scene showing the shop room after a black drum exploded, damaging the ceiling.

Eric Leighton death

A police photo shows some of the aftermath of the explosion that killed Eric Leighton, including what is left of the drum he was working on. (Ottawa Police)

Leighton had been using a hand grinder to cut the barrel, which also once held a cleaning solvent, when a spark ignited gas inside of it.

Paramedics said Leighton could not be resuscitated

Paramedics who arrived said they found the teenager on the shop floor and said they could not resuscitate him.

The emergency room doctor said despite efforts to revive a heartbeat, the trauma to the head had left Leighton brain dead.

The inquest is expected to last two weeks and hear from at least 14 witnesses, followed by recommendations from the jury on how similar incidents could be prevented.

Eric Leighton was using hand grinder when explosion happened

Eric Leighton was using this hand grinder on the drum when the explosion happened. (Ottawa police)

Leighton's family have also filed a $400,000 lawsuit against shop class teacher Scott Day and the Ottawa Catholic School Board.

They allege both the teacher and board should have seen the danger of doing "hot work" on a barrel that once held combustible gas.

Day is scheduled to testify on Tuesday.