Santa Ana, California (CNN) -- Prosecutors have yet to decide whether they will seek the death penalty in their murder case against Itzcoatl Ocampo, a 23-year-old Iraq war veteran accused of stabbing four homeless men to death in California, Orange County District Attorney Tony Rackauckas said Tuesday.
Ocampo was charged Tuesday with four counts of first-degree murder and those charges carry the special circumstances of multiple murders, lying in wait and use of a deadly weapon, the prosecutor said.
The minimum sentence for the four murder charges is life imprisonment without the possibility of parole, and the maximum penalty is death, prosecutors said. Prosecutors will be considering whether to seek a death sentence, Rackauckas said.
Ocampo's arraignment is scheduled for Wednesday, he added.
"I'm not prepared to discuss a possible motive," Rackauckas said. "We know he had selected others" to kill, he said.
"The fact that the violence was escalating and it was becoming more brazen suggested to me he wasn't done. He appeared to me he liked the press coverage," the prosecutor told reporters.
"This 23-year-old was a vicious killer. He had on his mind to kill people, and he followed through on that, and he was a monster, and he was a terrible threat, particularly to the homeless people in our county," Rackauckas said.
"We have no indication whatsoever that he was mentally ill," the prosecutor said.
The first victim was stabbed more than 40 times, Rackauckas said, and "in each of these cases, the violence, the number of stabbing wounds of each victim, increased."
The murder weapon was a 7-inch, heavy-gauge Ka-Bar Bull Dozier knife, "which went through bone without chipping or breaking the blade," the prosecutor said, showing the knife to reporters.
"It looks like a military kind," he added about the knife.
Ocampo was arrested Friday night after he allegedly stabbed a transient to death, Anaheim Police Sgt. Bob Dunn said.
The arrest left those who know Ocampo confused.
A friend said Monday that "something happened" to Ocampo after he came back from serving in the Marines in Iraq.
Attached to the 1st Marine Logistics Group at Camp Pendleton, California, Ocampo was a corporal and a motor vehicle operator, serving in the Marines from July 2006 until July 2010, and was deployed to Iraq for six months in 2008, according to Marine service records. He received an Iraq campaign medal with one star, a Global War on Terrorism Service Medal and a National Defense Service Medal, records showed.
When Ocampo returned from Iraq, "he was a little bit more serious, it seemed like there was something on his mind," said Brian, who lived a few doors down from Ocampo and asked to be identified only by his first name. "He seemed really depressed and down, and things in his life weren't looking that well."
The suspect's father, Refugio Ocampo, himself homeless, told the Orange County Register that his son worried about his safety.
Refugio Ocampo, who lost his home in 2008 and now lives in the cab of a semi-trailer parked in Fullerton, said it was hard to believe his son could be involved in the killings.
"I saw him so many times giving the last money he had in his pocket ... to the homeless, to the people that (are) asking for some help. ... My son's always been a role model," Refugio Ocampo said in a video interview posted on the newspaper's website.
Refugio Ocampo, 50, told the Register that his son, worried about his father's safety, had recently pointed out an FBI warning about homeless men being killed in the area.
Refugio Ocampo said his son returned from serving in Iraq a changed man. He started talking about things that "didn't make any sense," like the end of the world.
"They killed the person he was," the father told the Orange County Register. "And that's the only possibility I can think of that he would do something like that."
Brian, the neighbor, described Itzcoatl Ocampo as a "fun ... ordinary kid" when they grew up together in Southern California, adding "he was really looking forward to the service" after graduating from high school.
"When you were around him, you had fun," the suspect's high school classmate said.
But like Ocampo's father, Brian said he noticed some recent changes in the young man, including when the two met up a few times last summer.
"I knew he wanted to see (combat) action, and I knew he enjoyed it," Brian told CNN, noting he'd lent his friend a self-help book. "Then something happened, and I just don't know what."
Brian added that he had a hard time believing that his friend could be responsible for the killings.
"I would never have guessed this," he said.
Norberto Martinez, a family friend who lives with the veteran's uncle, mother and two siblings in Yorba Linda, California, described the former Marine as mild mannered, but the friend avoided talking about the war because it upset Ocampo. Martinez and Ocampo watched ball games on television and held conversations during walks, Martinez said.
"He was never angry or furious," Martinez said. "We were surprised" to learn that he was a suspect in the four murders."
"I wouldn't talk to him about Iraq," Martinez said. "Whenever he talked with me, he was normal."
Authorities had been searching for a serial killer after three homeless men were stabbed to death over 10 days last month in the Orange County cities of Anaheim, Brea and Placentia.
Police had released a surveillance camera image of the suspect taken December 20. It shows the suspect standing a few feet from the victim, who appears to be lying on the ground next to a strip mall, according to police.
Investigators urged the homeless to stay in groups and avoid sleeping in dark and secluded areas.
The four slaying victims are James McGillivray, 50, who was killed December 20; Lloyd "Jimmy" Middaugh, 42, who died December 27; Paulus "Dutch" Smit, 57, killed on December 30; and John Berry, 64, who was stabbed to death January 13.
CNN's Michael Martinez in Los Angeles, Linda Hall in Santa Ana, Jaqueline Hurtado and Gabriel Falcon contributed to this report.