Authorities say Itzcoatl Ocampo of Yorba Linda was found ill by deputies on Wednesday night. He was taken from Central Jail to Western Medical Center in Santa Ana, where he was pronounced dead at about 1:30 p.m. Thursday.
The cause of death has not been released. No further information was immediately available on the death.
Ocampo was charged with six felony counts of murder with special circumstances and was being held without bail. His trial was set for May 2014. Prosecutors planned to seek the death penalty.
The 24-year-old served an eight-month deployment in Iraq in 2008 and was discharged in 2010. According to secret grand jury testimony, Ocampo reportedly joined the Marine Corps because "he needed to kill." Ocampo said he was disappointed when he did not see any combat during a tour of duty because he became a truck driver instead of a killer, court documents stated.
The homeless murders took place in Orange County over a three-week period starting in December 2011. Ocampo was arrested following the stabbing death of the fourth homeless man outside a Carl's Jr. restaurant in Anaheim on Jan. 13.
Ocampo was also charged with stabbing to death a mother and son in Yorba Linda in October 2011. Unlike the homeless victims, authorities allege Ocampo knew 53-year-old Raquel Estrada and her 34-year-old son Juan Herrera through Estrada's younger son.