SANTA ANA, Calif. — A former Marine already accused of killing four homeless men was charged Monday with two new counts of murder in the stabbing deaths of his high school friend's mother and older brother.

Itzcoatl Ocampo, 23, was charged in the stabbing deaths of 53-year-old Raquel Estrada and her son 34-year-old Juan Herrera. Charges were dropped Friday against Estrada's son Eder Herrera, 24, a former schoolmate of Ocampo.

Orange County District Attorney Tony Rackauckas said Ocampo didn't only prey on the homeless.

"The homeless people killed were all easy victims but he clearly wasn't limited to that," Rackauckas said to reporters. "He's just a monstrous serial killer."

Ocampo appeared in court but his arraignment was postponed to March 16 after his attorney sought more time to look into the new allegations.

"I'm shocked as to these charges," said defense attorney Randall Longwith.

Estrada and her son were killed in their home in October, and Herrera was arrested shortly after, but Rackauckas said "significant evidence" was found linking Ocampo to the mother and son killings after DNA samples were taken from his home.

Rackauckas, however, has said Estrada has not been conclusively eliminated as a suspect, and he also must face a hearing to determine if he can remain in the country after telling immigration officials he was here illegally.

Herrera's brother Juan was found stabbed more than 60 times, his body left in a hallway of the family's rented three-bedroom house on a quiet residential cul-de-sac in upscale Yorba Linda. His mother was stabbed more than 30 times, authorities said.

The stab wounds were similar to those suffered by the four homeless men bludgeoned to death in December and January, a series of killings that terrorized the homeless population of Orange County. Each of the four men was stabbed more than 40 times with a weapon believed to be a 7-inch, fixed-blade, military-type knife, authorities said.

Victim James Patrick McGillivray, 53, was stabbed Dec. 20 near a shopping center in Placentia; Lloyd Middaugh, 42, was found Dec. 28 near a riverbed trail in Anaheim; Paulus Smit, 57, was stabbed to death outside a Yorba Linda library on Dec. 30; and John Berry, 64, was stabbed to death on Jan. 13, the day Ocampo was arrested.