Bronson Nunuha of Maui died in his prison cell at the Saguaro Correctional Center in Eloy.
In a statement from Corrections Corporation of America (CCA), the private company that owns the prison, spokesman Steve Owen said Nunuha was found in his cell "with undisclosed injuries, the apparent victim of an assault."
He said staff found Nunuha about 9:30 in the morning. Efforts to revive him failed.
"At this point I don't know how many people are suspected involved in this incident. I'm awaiting further information from CCA officials," Hawaii Public Safety director Clayton Frank said.
Frank wouldn't say how Nunuha died but sources said he was stabbed.
Nunuha shared a cell with another inmate.
"I believe at the time of this incident the housing unit was not on lockdown," Frank said.
"The inmates are able to close their doors if they so choose to do that."
Saguaro is a 1900-bed facility located between Phoenix and Tucson. It was built for Hawaii inmates guilty of crimes from theft to murder.
After Nunuha's death the prison was put into lockdown.
Frank could not comment on whether Nunuha's death was gang related but said Saguaro has gang activity.
McKay Schwenke deals with Hawaii's youth gang offenders.
"When they become adults and they're incarcerated in adult prisons, sometimes they gravitate toward certain types of groups for safety," he said.
"Anytime we have a person that is seriously injured or, in this case, a death occurs it's always serious for us," Frank said.
He said he's sending a team of investigators and a gang intelligence coordinator to Saguaro.
He said Nunuha's family has been notified of his death. He was serving a five-year sentence for second-degree burglary. He was due to be released Oct. 31.