The father of a 5-year-old boy killed in 1975 is vowing to murder his son's killer if the man is released, as scheduled, several years early from a 40-year sentence.

John Foreman said in an interview Monday with WPRO-AM radio that he will kill convicted murderer Michael Woodmansee "as aggressively and as painfully" as he killed his son if Woodmansee is released from prison early.

Woodmansee, who was 16 years old at the time, kidnapped and killed Jason Foreman in 1975 in South Kingstown, R.I. He confessed and was convicted of second-degree murder eight years later.

Jason Foreman was presumed to be missing until 1982, when Woodmansee tried to lure another boy into his home. The boy escaped and police began to question Woodmansee about Foreman's disappearance.

Authorities found the boy's skull and bones on Woodmansee's , along with a journal that detailed the gruesome killing. John Foreman told the radio station that Woodmansee wrote about eating his son's flesh in the journal.

"That's what he thinks about. That's what is still on his mind I'm sure, if gets out again, to do this again," Foreman said.

Woodmansee pleaded guilty to second-degree murder in 1983 and was sentenced to 40 years in jail. This was part of a plea bargain meant to spare the Foreman family from hearing the details of their son's death.

But Woodmansee is set to be released 12 years earlier than was previously expected, the Providence Journal reports, sparking outrage from the Foreman family.

"I do intend, if this man is released anywhere in my vicinity, or if I can find him after the fact, I do intend to kill this man," Foreman told the radio station.

Amy Kempe, a spokeswoman for Attorney General Peter Kilmartin, said in a statement Monday that he was concerned and outraged about Woodmansee's scheduled release, and said he was urging the Department of Corrections to consider all avenues available to keep him behind bars. Kempe said the office would work with the Department of Corrections to review all legal options available.

Patricia Coyne-Fague, chief legal counsel for the Department of Corrections, said Monday she had not yet heard from the attorney general's office, but that typically the only way an inmate can lose good time he's earned for early release is if he misbehaves.

She explained that Woodmansee is eligible for early release under a longstanding law, first put in place in 1872, and last significantly changed in 1960. That law allowed Woodmansee to earn up to 10 days off his sentence for every month he behaved. Because he also had a job in prison, he was eligible to receive up to two additional days per month off his sentence for every month he worked at least 15 days, she said.

For his own protection, Woodmansee served nearly all of the last 28 years of his sentence in prisons in Massachusetts instead of Rhode Island, but returned to the state last week, the newspaper said.

Click to read more on the killer's early release from The Providence Journal

Click here to read more on this story from MyFoxBoston.com

The Associated Press contributed to this report