FULLERTON, Calif. (KABC) -- The FBI is joining the investigation in the case of a homeless man who died following a struggle with Fullerton police.   Officers confronted 37-year-old Kelly Thomas July 5 after getting reports of a man burglarizing cars. They said Thomas became combative..   Thomas, who was schizophrenic, was hospitalized with severe head and neck injuries and died several days later.   It's still unclear what force was used on him.   Orange County Supervisor Shawn Nelson asked the Department of Justice to investigate.   If you have any information regarding this case, you're urged to call the Orange County district attorney's office at (714) 347-8813.