A detective who swore he wouldn’t rest until the Gilgo Beach murderer was caught and punished told The Post this week he will finally retire — and is happy to know the monster will be locked away for the rest of his life.

Detective Donald Blatchford, who’s been with Maine’s Scarborough Police Department for 37 years, said Rex Heuermann’s guilty plea was a much-welcomed end to a grueling 16-year saga.

“In these most horrific cases, I can assure you everybody that worked on it felt very satisfied after he made his plea,” Blatchford said. 

The Army vet was brought onto the case in June 2010 when Scarborough resident Megan Waterman, 22, vanished after leaving a Holiday Inn Express on Long Island. 

Waterman’s remains were found along Ocean Parkway near Gilgo Beach that December – along with those of Maureen Brainard-Barnes, 25; Amber Lynn Costello, 27; and Melissa Barthelemy, 24 — who later were dubbed the “Gilgo Four.”

“After investigating her being missing for as long as she was, there was an assumption that it could end up being that way,” Blatchford, 60, said. 

“Obviously, we didn’t know it was going to end up involving other individuals as well.”

Heuermann, 62, was arrested in 2023 and pleaded guilty Wednesday to murdering seven sex workers over the span of decades, including Valerie Mack, 24, Jessica Taylor, 20, and Sandra Costilla, 28. 

He also admitted to slaughtering Karen Vergata, 34. He was not charged with Vergata’s death as part of his plea deal, but agreed to cooperate with the FBI’s Behavioral Analysis Unit.

While he had testified before a grand jury for charges to be brought against Heuermann, Wednesday was the first time he laid eyes on the killer as Heuermann pled guilty in Suffolk County court.

“It did kind of appear that he had a little bit of a smile — it was almost like he was enjoying reliving what he had done,” Blatchford said. 

“It was a bit of a shock. I was taken aback a little bit by the lack of emotion from him.” 

Heuermann’s attorney did not immediately respond to a request for comment. 

Victims’ loved ones said they were relieved the harrowing ordeal is nearly over, as Heuermann is due back in court for sentencing on June 17. He is expected to be hit with three life sentences.

“On behalf of Megan, we definitely accept the plea,” Waterman’s aunt, Elizabeth Meserve, told reporters after the hearing. Waterman’s daughter, Liliana Waterman, 19, shared the same sentiment.

For Blatchford and other investigators, the guilty plea marked the end of years of uncertainty. 

“It is definitely the longest process in my history of doing any kind of investigation,” Blatchford said.

Suffolk County District Attorney Raymond A. Tierney commended Blatchford for his diligence.

“He held off his retirement to see this case through,” the DA said during a news conference. 

“I made that commitment and intended to keep it,” Blatchford explained, adding this time next year, he’ll have likely moved on from the department. 

“I’ve described it before as living other people’s worst nightmares … and it does wear on you. It’s definitely stuff that sticks with you.”