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Los Angeles Times via AP) MANDATORY CREDIT, Al Seib
Terri Gaspar places flowers at Madrona Elementary School, where a homemade model rocket exploded, killing at least one high school student, in Thousand Oaks, Calif., Tuesday, April 5, 2016. Investigators will try to piece together a homemade model rocket to figure out why it exploded at a Southern California school, killing a high school student and sending his friend to a hospital with minor injuries, authorities said Tuesday.

THOUSAND OAKS, Calif. — Investigators will try to piece together a homemade model rocket to figure out why it exploded at a Southern California school, killing a high school student and sending his friend to a hospital with minor injuries, authorities said Tuesday.

The two senior honor students apparently were experimenting with "some sort of chemical combination" before the accidental blast Monday evening in the courtyard of an elementary school in their neighborhood, said sheriff's Capt. Garo Kuredjian. He said the fuel was not propane, as officials initially reported.

Investigators hoped to determine what chemicals were used as well as the size and shape of the rocket that blew apart, Kuredjian said.

Boy Scouts and Girl Scouts were meeting at Madrona Elementary in Thousand Oaks when the school suddenly shook from the blast.

"Nobody could really discern what it really was because it was a huge explosion. It was just all at once. Just a 'boom!'" Tammy Coburn, who was attending the meeting, told reporters.

A nurse who heard the blast helped the critically injured victim, an 18-year-old man who died hours later at a hospital, Ventura County Fire Capt. Mike Lindbery said. His 17-year-old friend was hospitalized with minor injuries and expected to survive.

The experiment wasn't sanctioned by any school, and the victims likely were using the courtyard because it's an open space, Kuredjian said. Classes at the elementary school northwest of Los Angeles were canceled Tuesday.

The county coroner's office identified the man who died as Bernard Moon of Thousand Oaks. An autopsy was planned. The injured boy was not named.

Both were seniors at nearby Thousand Oaks High School.

"They're ... honor students. Amazing, brilliant kids," Coburn said.