The shooter or shooters hit and killed a security officer, Pat Underwood. Another officer was also injured.
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The FBI is looking for a white van that's either a 1997-2002 Ford E-250 or E-350. They say the van didn't appear to have license plates attached at the time of the shooting.
The agency is asking business owners and residents in the downtown area to check surveillance footage from the night of Friday, May 29 between 7 p.m. and 11 p.m. for any sign of the van. The area they're interested is between Grand Avenue and 7th Street to the north and south, and Market and Webster streets to the west and east.
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They are also asking anyone who may have information on the firearms used in the attack to come forward.
Anyone with information is asked to contact 415-553-7400.