WOODLAND, Calif. (AP) — A California father about to be freed from prison has been taken into custody in connection with the decades-old killings of five of his infant children in a case a sheriff said has haunted his agency for years.

Paul Perez, 57, a convicted sex offender with a 20-year criminal history, was charged in the deaths of the children born between 1992 and 2001, authorities announced Monday, the same day he was supposed to be released from a state prison in Delano on unrelated charges.

Yolo County Sheriff Tom Lopez said the case reflects an “unspeakable evil,” though a motive has not been determined.

A homicide investigation was launched in the spring of 2007 after a fisherman found the badly decomposed remains of a 3-month-old boy recently identified as Nikko Lee Perez.

An autopsy determined the infant died of blunt force trauma. He had a fractured skull and before death had sustained rib and other fractures that had started to heal by the time he was killed.

Investigators submitted the baby’s DNA to the U.S. Department of Justice in hopes of obtaining an identity. They were able to determine Nikko’s identity.

With the new information, investigators determined Nikko had siblings: one who is known to be dead and three others whose remains have not been found.

Perez was charged with five counts of premeditated murder with special circumstances of lying in wait, torture and multiple victims.