OAKLAND — A private security guard was shot in the head but finished his shift and went home before being hospitalized and later dying, police said Thursday.

Demetrio Baltazar, 59, of Oakland was shot Sunday afternoon as he sat in a security vehicle at an East Bay Municipal Utility District maintenance facility where he worked.

Investigators believe Baltazar was hit by a stray bullet from a shooting that took place nearby.

Oakland police Sgt. Jim Rullamas said it’s not clear why Baltazar didn’t seek treatment.

“He knew that he was hurt,” Rullamas said. “We have him on a surveillance video holding a towel to his head and cleaning up. I can’t answer why he didn’t call 911 or go to the doctor. I just don’t know.”

Baltazar, who worked for Guardsmark Security which contracts with EBMUD, finished his shift and drove himself home. There, Baltazar’s 11-year-old son tried to help him with his injury, Rullamas said.

Baltazar’s condition worsened and he was taken to the hospital but did not recover, dying on Wednesday.

Meanwhile, another guard called police after finding blood inside the security vehicle.

Friends and family didn’t describe Baltazar as being “macho,” or likely to shrug off a serious injury, Rullamas said. “Everybody said he’s just a real friendly, dedicated kind of worker.”

Police were investigating the shooting that was believed to have resulted in Baltazar’s death, Rullamas said.

“This is just a man who is a completely innocent victim,” he said.

Reached at home, Baltazar’s wife declined to comment. In addition to his wife, Baltazar left three sons, Rullamas said.

Officials at New York-based Guardsmark were “deeply saddened” by his death, said Stephen Kasloff, a company vice president. He said company officials are in touch with the family.

Gary Leviton, also a Guardsmark vice president, said he did not know exactly how long Baltazar had worked for the company but it had been some time. “He was very well-respected by his fellow security officers, by his supervisor,” he said.

“This is a real loss to Guardsmark and a loss, obviously, to his family and friends but really a loss to the community as well,” Leviton said. “This is a good guy.”