Commander Greg McEachern told a news conference Friday that investigators have not yet determined Jimmy Lam's motive, but "the shooting appeared purposeful and targeted."
The 38-year-old Lam shot and killed three fellow drivers and wounded two others before killing himself in front of police in the June 14 shooting rampage at a San Francisco UPS warehouse.
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McEachern said Lam was armed with two stolen weapons.
He described one as an assault pistol stolen in Utah, which was used in the killings. Lam also had a semi-automatic handgun stolen from Napa County, California.
McEachern said it was not known who stole the weapons.
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