According to the San Leandro Police Department, officers arrested Oakland resident Akbar Bey, 27, Tuesday morning.
Police said Bey confessed to detectives about his involvement in the case, including shooting the victim, identified as John Mendez, in the back of his head before he stole the messenger bag containing cash.
Bey used to be a guard who worked with Mendez on the same route before he was fired for stealing money, San Leandro police said.
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On Wednesday, Bey was officially charged by the Alameda County District Attorney's office for murder.
This investigation is ongoing.