A knife sharpener and a book titled "The Most Notorious Crimes in American History" were found inside Itzcaotl Ocampo's home, court documents show.
Ocampo remains jailed without bail. He is on psychological watch in a solitary cell apart from other inmates.
Ocampo's arraignment is scheduled for Feb. 17.
Prosecutors have filed four felony murder charges against Ocampo.
The charges include the special circumstance of multiple killings and allegations that they were committed while lying in wait and with the use of a deadly weapon.
Ocampo faces a minimum of life in prison without the possibility of parole if convicted on all charges.
He is eligible for the death penalty, if convicted, but prosecutors have not decided whether to pursue it.
Authorities believe Ocampo stalked his victims and likely had more victims planned, according to Orange County District Attorney Tony Rackauckas.
They think Ocampo liked the media attention and targeted his last alleged victim, 64-year-old John Berry, after he was interviewed for an article in the Los Angeles Times.
"The evidence is going to show that the defendant specifically sought out this victim for participating in this article, that the defendant relished the media attention of the crime and he stalked the victim until he got his prey," Rackauckas said.
Berry had called police, saying he thought he was being followed, according to Anaheim Police Chief John Welter.
"It's unfortunate that we did not get to him before the suspect did," Welter said, adding that police received hundreds of tips in the case.
Ocampo was arrested about a quarter-mile from the scene of Berry's murder in Anaheim.
Witnesses who saw the stabbing chased Ocampo down before police arrived and took him into custody.
Investigators believe that Ocampo used a 7-inch Ka-Bar Bulldozer knife in the murders, Rackauckas said.
Officers found the knife, which was capable of cutting through bone, near the spot where Ocampo was arrested.
Police have not pinpointed a motive, but prosecutors say Ocampo became more "brazen" and violent with each of his alleged attacks.
One of his alleged victims was stabbed 50 times, authorities said. In a later attack, the victim was allegedly stabbed more than 60 times.
Family members describe Ocampo a generous man, who was inspired to join the Marines after the 9/11 attacks.
But they say something changed in him after his deployment to Iraq in 2008.
After he left active duty in 2010, Ocampo returned to Orange County to find his father living under a bridge.
Ocampo moved into the family's rented home in Yorba Linda, but he suffered from depression, hand tremors and hallucinations, his brother, 17-year-old Mixcoatl Ocampo said.
His paranoid behavior worsened, according to the family, after a close friend from the Marines was killed in combat in Afghanistan in 2010.
The Veteran's Administration diagnosed Ocampo with psychological problems, according to his brother.