The officials said the semi-automatic weapon appears to have been built with parts that are legal to obtain, but put together make the rifle illegal in California.
Investigators found a drill press in 23-year-old John Zawahri's bedroom among other materials that indicate he likely assembled the weapon.
The drill press is used to help finish building the rifle by drilling holes in the lower receiver. A lower receiver that is only 80 percent complete can easily be purchased, and because it is not complete a person isn't required to go through a background check nor does it need to have a serial number.
In California such weapons require a "bullet button" kit, which needs to be added to a lower parts kit to make it legal. The bullet button kit modifies the weapon so that a separate tool must be used to release a magazine and reload it; without such a modification a person can press a button to release the magazine.
Zawahri was carrying 1,300 rounds of ammunition in magazines that were capable of holding 30 rounds each.
Such high-capacity magazines are illegal to purchase, sell or transfer in California. Possession is not illegal.
Zawahri's 15-minute midday rampage Friday spanned a mile between his father's home, where his father and brother were shot to death, and Santa Monica College, where police shot him to death in the library. Along the way he fired at vehicles and strangers, fatally wounding three people. One other person was seriously wounded and two others had minor injuries.
Zawahri's last reported contact with law enforcement was seven years ago when bomb-making materials were found at his house during a search prompted by threats to students, teachers and campus police officers at Olympic High, a school for students with academic or disciplinary issues.
Retired police officer Cristina Coria, who helped serve the search warrant, said Zawahri was hospitalized for psychiatric evaluation at the time. She didn't know the outcome of the evaluation.
But once a person is held for such an exam, they cannot access or possess firearms for five years.
In the case of Zawahri, that prohibition would have expired in 2011.
The Santa Monica-Malibu school board was briefed at the time by school administrators after police found Zawahri was learning to make explosives by downloading instructions from YouTube, school board member Oscar de la Torre said.
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