The incident happened Wednesday evening at an apartment complex on Central Avenue, according to the Garden Grove Police Department.
Officers initially responded to a burglary in progress but when they arrived, they found the suspect being subdued and held down by residents in the area.
The witnesses directed officers to a residence where they found the victim, an 87-year-old man, unresponsive. Chuong Pham was taken to a hospital where he succumbed to his injuries.
Pham's sons told Eyewitness News he had been suffering from dementia. They said it was common for people in the community to walk him home when he lost his way.
For some reason, they say the suspect, 26-year-old Mario Brancato, came home with their father. A surveillance camera in the family's living room captured the two calmly talking.
The sons said they had never seen Brancato before, but he stuck around for about an hour as their father showed the suspect pictures and other things in the home.
When one of the sons saw the footage, he ran home worried. But when he arrived, it was too late. He found the two in a bedroom, his father tied down and bleeding from the head.
The suspect then ran off and are grateful to neighbors who helped hold him down until police arrived.
According to Orange County court records, Brancato was on probation and sentenced in August to 30 days in jail after pleading guilty to two offenses from 2022, vandalism and DUI.
The victim's children now have a warning for families caring for elderly parents.
"Don't talk to strangers. Don't let people in the house. When you have an old dad or mom that stay home by themselves, take care of them," said Khan Pham.
Authorities said they don't have a known address for Brancato and they don't know where he's from so they believe he's a transient.