Arriving officers found two men lying in the roadway, police said.
One victim was pronounced dead at the scene. The second was taken to a hospital, where he later died, police said. Investigators have not released the victims' names pending notification of their families.
No arrests have been made, and police have not determined a motive for the double homicide.
Police are asking witnesses to come forward to assist the investigation.
Anyone with information is urged to contact Union City Police Department at (510) 471-1365.
Anonymous tips can be communicated by calling (510) 675-5207.
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