"It's something that can be used as a noisemaker or actually shoot a projectile," Nance said. "We don't know exactly what was in it or if there was a projectile."
Investigators hope an autopsy will show what combustible material may have been used and how it was ignited, Nance said.
Family friend Danielle Kunsman said Ostberg was the best friend of her son, Cameron, and previously had demonstrated his miniature cannon for others. She said her son told her he has seen Ostberg fire only bits of aluminum foil.
Kunsman said Ostberg was "very talented, mechanically." Despite leaving Bear River High School in ninth grade, Ostberg had learned to repair neighbors' badly damaged electronic and motor appliances.
"He was willing to help anybody," Kunsman said. "He helped my daughter fix her TV. My husband gave him a chain saw that [was missing parts], and that kid got that thing running again. A neighbor said [Otsberg] was just out here, helping ... change his tires. If it was broken, he'd fix it."
Ostberg's 16-year-old brother was in the living room when the cannon fired but was not handling it and has few details as to what led to the shooting, Nance said. Ostberg's father also was in the house but was not in the room, Nance said. The boy's mother lives in Salt Lake County, he said.
Ostberg's former classmates at Bear River High School planned to dress in formal wear Tuesday in his honor, Kunsman said.