We're waiting for the autopsy, Joyce Hansen, administrative assistant to the chief of police in Tremonton, Utah, told ABC News.
But those results could take weeks, depending on the workload of the state medical examiner's office located in Salt Lake City.
Authorities said Ostberg was playing with the toy canon around 7:30 a.m. Monday in the living room of his Tremonton home, some 75 miles north of Salt Lake City.
A press release reviewed by the media noted that Ostberg was playing video games with his 16-year-old brother when the cannon accidentally went off. The boy was already dead when police arrived on scene. Ostberg's father, Allen Ostberg, was also home at the time of the incident.
ABC4 Salt Lake City reported that Ostberg's father was an Army veteran who had guns in the home. However, the father locked the guns and also taught his sons gun safety, according to that report. Investigators are still uncertain how a propellant got into the cannon.
Media reports describe the toy canon as having a six-inch metal barrel and a wooden base and the press release noted that it was a .50-caliber weapon designed to be fired with black powder.
Cameron Kunsman, who told the Deseret News that Robby Ostberg was his best friend, said the dead teen would frequently fill the cannon with black powder and fired tinfoil balls out of it.