Umatilla County District Attorney Dan Primus says 26-year-old Jayson Matthew Withers of Salem was pronounced dead Friday morning after being taken to a local hospital.
Primus says Withers and another inmate were assaulting a third prisoner in the yard at the Eastern Oregon Correctional Institution, and when they wouldn't stop, a tower guard fired one shot that hit Withers.
Withers has been in the prison since 2010 on negligent homicide, robbery, and identity theft convictions. He was due for release in four more years.
The prison was put on lockdown after the shooting.
Primus says the investigation continues. The guard and the other inmates were not identified.