Witnesses said six officers fought with Kelly Thomas, 37, a schizophrenic, and used a stun gun to subdue him July 5, the Los Angeles Times reported.
The incident sent Thomas into a coma and he died after being taken off life support.
A photo taken of Thomas at the hospital showed his face grotesquely swollen and covered with bruises and cuts.
A spokesman for Fullerton police said two officers approached Thomas at the bus depot after receiving a report of someone trying to break into parked cars.
Thomas tried to run from the officers, who found items in his backpack that didn't belong to him, the spokesman said.
The officers called for backup when Thomas resisted.