The San Diego East County Sheriff's Office homicide detectives are investigating the case, and have not released the woman's name.
She was found dead early Tuesday after a projectile struck her home in the Twin Lakes Resort mobile home park near the Town of Potrero, near the Mexican border, California Department of Forestry and Fire Protection Sgt. Mike Mohler said.
Investigators spoke with two men who said they were working on the cannon when it fired. One was injured in the firing and was hospitalized, Mohler said.