Sheriff's deputies responded to a call for police assistance at a homeless encampment near the Concord Avenue on-ramp to northbound Interstate Highway 680 at about 8 p.m., sheriff's spokesman Jimmy Lee said.
Contra Costa County Fire Protection District firefighters and medical personnel were the first to arrive at the scene, and had tried to revive Nagle. They called in deputies a short time later.
Nagle was pronounced dead at the scene.
Lee said Nagle, 47, a transient, had gotten into a physical altercation with another transient, 38-year-old James Riley, prior to his death.
The cause of the death is unknown, Lee said. An autopsy is scheduled for later Thursday.
Riley was taken into custody and booked into county jail on suspicion of murder, Lee said. He is being held on $1 million bail.
The case remains under investigation.