He was one of a handful of white inmates in a dorm with about 200 Latinos, violent gang members among them, records show. The gang members struck when a guard left to monitor an area about 75 feet away, where he couldn't see or hear those in McNamara's area.
Mike Cloud, another white inmate in the area, later testified that several gang members had the mistaken impression that McNamara was in jail on a child molestation charge, an offense that often results in retaliation from fellow inmates.
Cloud testified that when the guard left, the gang members singled out McNamara and got him down on the ground, then took turns jumping off a third-tier bunk and landing on his head. The attack went on for so long that "they got tired and took breaks before resuming," he said.
One of the inmates has since pleaded no contest to attempted murder and was sentenced to six years in state prison, according to a district attorney's spokeswoman.
Thadd McNamara didn't learn of the attack until days after it occurred. It was nearly a year before he saw his son. At first, jailers wouldn't grant him access while his son was in the hospital, he said. Later, Sean refused to see any family members -- a decision his father attributes to his altered mental state after the attack.
When McNamara finally saw Sean at a court hearing in the burglary case, he barely recognized him. Once a strapping six-footer, weighing 200 pounds or more, Sean McNamara was down to 130 pounds, his father said. His hair, once short, dangled to his shoulders and covered his face.
"Here was this little old man who was my son," McNamara recalled. He began to weep as he knelt next to his son at the defense table.
The burglary case was later dismissed because of the severe injuries Sean had sustained while in jail.
These days, Sean resides in an assisted-living facility. His father said he has no memory of the attack and has the intellect of a "third- or fourth-grader."
His dad routinely visits, often taking Sean to a nearby In-N-Out for burgers. But he said Sean's injuries have so severely altered his personality that he's like a different person.
"You miss him every single day. Even when he's there, you miss him," McNamara said. "There's just this big hole."
Despite his long career in law enforcement, McNamara said he's convinced the system failed his son.
"Somebody didn't do their job, and I damn near lost a son because of it," he said.