A California sex offender already in prison on unrelated charges has been accused of torturing and killing five of his own infant children over a nine-year span between 1992 and 2001, authorities said. Paul Allen Perez, 57, was arrested on five counts of premeditated murder Monday, days before he was due to be paroled from prison. According to California Department of Corrections and Rehabilitation records, Perez has been in Kern Valley State Prison in Delano since August 2010. He was arrested on the murder charges at Kern Valley, Yolo County Sheriff Tom Lopez said during a news conference Monday morning. “The fact that he was not allowed to walk out of that prison a free man cannot be overlooked,” Lopez said. Perez is accused in the deaths of Nikko Lee Perez, Kato Allen Perez, Mika Alena Perez, Kato Krow Perez and another infant named Nikko Lee Perez, who was born the year after the brother who shared his name. His murder charges include the special circumstances of lying in wait, torture and multiple victims. According to The Associated Press, he also faces charges of assault on a child under 8 and criminal enhancements due to his prior convictions. Lopez described the alleged slayings as acts of “unspeakable evil” that “ignited a resolve in the hearts of all involved to bring justice to the vulnerable and innocent victims of this case.” Perez, a convicted sex offender, became a murder suspect after investigators took a look at the cold case of a slain infant found March 29, 2007, in an irrigation slough just east of Woodland. According to the AP, the baby boy was wrapped in a Winnie the Pooh blanket and a layer of plastic before being placed in a metal cooler. The cooler was weighed down with heavy objects and submerged in the slough, located about 15 miles northwest of Sacramento. The infant, who was about 3 months old when he died, suffered a fractured skull and other blunt force trauma, the AP reported. He also had healing fractures, including broken ribs, that showed a pattern of abuse. Lopez said having the unsolved case on the books haunted investigators in the Yolo County Sheriff’s Office. The case remained unsolved until October, when DNA testing identified the boy as Nikko Lee Perez, who was born Nov. 8, 1996, in Fresno. The sheriff did not go into details of how Nikko’s DNA led to his identification. Fresno is about 190 miles from where the infant’s body was found. It was not immediately clear if his body was in the slough the entire nine years before it was found or if he was hidden elsewhere and dumped there more recently. Watch the news conference with Yolo County law enforcement officials below. “His identification is the result of extraordinary work done by the Yolo County Coroner’s Office and the California Department of Justice (Bureau of) Forensic Services’ Jan Bashinski DNA Laboratory in Richmond,” Lopez said. “What began as a single unsolved homicide has become so much more. “Sadly, we learned during this investigation that Nikko was not an only child.” Detectives discovered that Nikko had at least four siblings, including Kato Allen Perez, who was born in 1992 in Merced and was known to have died. Kato’s cause and manner of death were not detailed Monday morning. The whereabouts of the other three siblings -- Mika, who was born in 1995; the second Nikko, who was born in 1997; and Kato Krow, born in 2001 -- are unknown, but investigators believe they, too, are dead. “All are now believed to have been murdered as infants,” Lopez said. The fisherman who discovered Nikko’s remains in 2007 told the AP he has never forgotten what he saw that day. “When I opened that box, I was 99 percent sure it was a human body, but I wanted to hold onto the belief that maybe it wasn’t,” Brian Roller said. “When I saw one of the officers (at the scene) start to cry, I knew right then that what I was thinking was true.” Roller told the news agency he was relieved to learn the long-ago mystery had been solved. Lopez said while DNA testing provided the break in the case, it was the “human element” that ultimately led to Perez’s arrest. He said a team of investigators “determined to learn the truth” spent countless hours on the case. “While I am proud of the efforts of my investigators and coroner’s office, this is not a day that will bring joy to any one of us,” Lopez said in a written statement. “In my 40 years in law enforcement, I cannot think of a case more disturbing than this one. There can be no victim more vulnerable and innocent than an infant, and unfortunately this case involves five.” Lopez told reporters he could not answer questions Monday about the case, which is still extremely active. In particular, he said he could not discuss anything regarding additional family members of either Perez or the slain children. Yolo County District Attorney Jeff Reisig, who also spoke at Monday’s news conference, said Perez is eligible for the death penalty. His office is still reviewing the case to determine if it will seek it. The AP reported that Perez was first sent to prison in 1990, sentenced to two years for assault with the intent to commit a sex offense. He was released the following year, according to California’s sex offender registry. He has also served time for vehicle theft, possession of a deadly weapon by an inmate and fleeing while on parole, according to the AP. Anyone with information on Perez or the slain infants is urged to contact the Yolo County Sheriff’s Office at 530-668-5280 or 530-666-8282. Anonymous tips can be called in at 530-668-5248.