The California Department of Corrections and Rehabilitation said One Goh, 50, died at California State Prison-Sacramento on March 20. Spokeswoman Vicky Waters told The Associated Press a cause of death hasn't been determined.
KTVU reported that the Sacramento County coroner would determine the cause of death.
Goh was sentenced in July 2017 to seven consecutive life terms in prison plus 271 years to life without the possibility of parole for the murders at Oikos University. Police said Goh was angry with school administrators for expelling him and refusing to refund his tuition.
Goh pleaded no contest to murder charges after a judge declared him mentally competent to stand trial -- he'd been judged unfit in 2013 and 2015. Goh entered his plea two weeks later and was spared a possible death sentence in return.
The April 2, 2012, shooting claimed the lives of university receptionist Katleen Ping, 24, and students Doris Chibuko, 42; Lydia Sim, 21; Grace Kim, 23; Judith Seymour, 53; Sonam Choedon, 33; and Tshering Bhutia, 38. Three others survived gunshot wounds.
The Associated Press contributed to this report.