The suspect, 43-year-old One Goh, is accused of fatally shooting seven people and injuring three others.
Investigators found the gun in a waterway Friday morning around 5:30.
“Investigators have worked very hard on this case. Hopefully, this find will help the families find closure and bring this case to justice,” said Oakland Police Chief Howard Jordan, in a news release.
Goh was charged Wednesday with seven counts of murder and three counts of attempted murder.