WOODLAND, Calif. (AP) — A California man has been arrested in the decades-old killings of five of his infant children, a case that the sheriff said: “has haunted my agency for years.”

“I cannot think of a case more disturbing than this one. There can be no victim more innocent than an infant and unfortunately, this case involves five of them,” Yolo County Sheriff Tom Lopez said.

Paul Perez, 57, of Delano was arrested at a state prison days before he was expected to be released on unrelated charges. He was arrested in the deaths of his children born between 1992 and 2001.

District Attorney Jeff Reisig said the killings involved lying in wait and torture, though he did not immediately reveal other details.

“The investigation in this case uncovered a deeply disturbing and chilling case of infanticide,” he said.

Authorities started investigating after the decomposed remains of the child recently identified as 3-month-old Nikko Lee Perez were found in a sealed, submerged box by a fisherman in 2007 in an irrigation slough. Those remains were identified in October using new DNA techniques that authorities have begun applying widely in the wake of other recently solved cases. Department of Justice officials checked the database weekly for direct matches.

“Sadly, during this investigation, we learned that Nikko was not an only child,” the sheriff said.

The infant’s body was found on March 29, 2007, in an irrigation canal popular with anglers looking for carp or catfish. A fisherman named Brian Roller told local news reporters at the time that he shot an arrow into the murky water, and it struck a box buried under the mud. When he hauled the box out of the water and opened it, he found stones used to weigh it down and the baby’s badly decomposed body wrapped in a blanket.

Authorities said a decomposed diaper was also in the box, indicating the infant had been cared for at some point.

Investigators immediately treated the case as a homicide and submitted the baby’s DNA to the U.S. Department of Justice in hopes of obtaining an identity. They also enlisted the help of a forensic anthropologist, who determined the remains were those of a boy about one month old who died of blunt force trauma.

The remains of three of the children have not been found.

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