Christopher Lacy shot Youngstrom on the shoulder of Interstate 680 in Alamo last September. The Contra Costa County Sheriff's investigation looked into the 36-year-old gunman's background.
Investigators found that Lacy suffered a mental breakdown while attending college and was diagnosed with bipolar disorder. The handgun he used during the freeway attack was lawfully bought and registered.
And investigators say they found no indication that Lacy was going to kill law enforcement officers.
Lacy was killed by Youngstrom's partner, Officer Tyler Carlton.