A former Marine who was awaiting a death penalty trial in the deaths of six people, including four homeless men, died after ingesting something while in his jail cell, his defense attorney said Friday.

Attorney Michael Molfetta would not say what his client ingested but said the death raises serious questions about how much supervision 25-year-old Itzcoatl Ocampo was getting from Orange County sheriff's deputies. The substance was not drugs, according to Molfetta, who says he was briefed on his client's death .

Ocampo was found in his single-man cell Wednesday and taken to a hospital where he was pronounced dead at around 1:30 p.m. Thursday, said Orange County sheriff's Lt. Jeff Hallock.

No cause of death was immediately given, and officials said an autopsy will be performed in the coming days. The Orange County district attorney's office will investigate the death.

Autopsy results by an independent pathologist are expected by the middle of next week, Hallock said.

"If you spend three minutes with Ocampo, you are acutely aware of the fact that he has some mental issues. They were severe and they were obvious and they definitely were not contrived," Molfetta said. "With that being said, this was a guy who should have garnered the highest level of scrutiny ... and it wasn't done."

Hallock declined to comment on inmate supervision except to say jail deputies are required, in general, to walk by each inmate's cell a minimum of once an hour.

Prosecutors alleged that the native of Mexico stabbed four homeless men in what they called a serial thrill-kill rampage in late 2011 and early 2012. Ocampo targeted his homeless victims because they were vulnerable and because he felt they were a blight on the community, authorities said.

He was charged in January 2012 with four counts of murder and special allegations of multiple murders and lying in wait and use of a deadly weapon. Three victims were stabbed more than 40 times each with a single-edged blade at least 7-inches long. In one instance, prosecutors, said, Ocampo selected as his next victim a homeless man who was featured in a Los Angeles Times story about the killings.

Ocampo, who was discharged from the Marines in 2010, also was facing murder charges in the deaths of a school friend's mother and brother in October 2011.

At the time, his family said Ocampo was a troubled man after he returned from Iraq in 2008 and went to visit the grave of a friend who died in combat in Afghanistan twice a week. He also began drinking heavily and he suffered from headaches and hand tremors, they said.

His parents separated in 2010 and his father became homeless after losing his job. During the killing rampage, Ocampo visited his father on the streets and warned him about the dangers of being homeless, even showing him a picture of one of the dead men.