A bus rider in San Diego has been charged with first-degree murder after he broke up a fight between a male and female passenger, then forcibly restrained the man for several minutes, killing him, cops said.

Edward Hilbert, 55, was arrested early Sunday for allegedly killing the 28-year-old man aboard the downtown city bus in California.

According to the San Diego Police Department, officers were called to the area of 1400 F Street for a disturbance on a bus operated by the Metropolitan Transit Service shortly after 8 p.m. Saturday, reported CBS8.

When cops boarded the bus, they discovered the 28-year-old man who had been restrained lying motionless on the floor.

The victim was given first aid and taken to a nearby hospital, where he was pronounced dead within two hours. His name has not yet been released pending the notification of next of kin.

Homicide detectives have determined that the victim had gotten into a fight with a woman on the bus, and that the 6-foot-3, 234-pound Hilbert intervened and held down the man for several minutes, according to Lt. Steve Shebloski with the San Diego Police Department.

It was not immediately known if any of the three previously knew each other.

While the victim was being restrained by Hilbert, he passed out and experienced medical distress, authorities said. His cause of death remained unknown as of Monday morning.

Hilbert is being held without bail in the San Diego County jail. He is scheduled to be arraigned on a charge of first-degree murder Wednesday afternoon, authorities said.