Rex Heuermann stood calmly in a dark suit and tie, his hands and ankles shackled, as the families of the women he murdered watched from the sidelines.

His estranged wife Asa Ellerup, 62, and daughter Victoria, 29, were also in the courtroom on April 8 as the 61-year-old architect finally pleaded guilty to a decades-long reign of terror that haunted Long Island.

It was the moment the victims’ families had waited years for.

The married murderer confessed to killing and dismembering eight women, from his first victim Sandra Costilla in 1993 to his last in 2010, after spending decades hiding in plain sight and living a chilling double life.

On the other side of the case was Suffolk County District Attorney (DA) Raymond Tierney, the prosecutor who helped put the Gilgo Beach Serial Killer behind bars.

Now, nearly three years after Heuermann’s arrest in July 2023, Tierney has spoken exclusively to the Daily Mail about coming face to face with the diabolical killer - and how the bombshell plea agreement unfolded.

The negotiations began in the summer of 2025, but were not made public until March.

While Tierney said only Heuermann knows why he changed his mind, he believes the killer would not have confessed and avoided trial if the case against him had not been so strong.

‘If this was a weak case, I don’t think he would have pleaded,’ the DA said. ‘He considered it. We were ready to go to trial.

‘I know his defense team was ready to take the case to trial, but for whatever reason, he made the decision. Perhaps he will give us more insight during the sentencing.’

As part of the plea negotiations, Tierney and Heuermann sat down and met face-to-face.

During their face-to-face meeting, Tierney said Heuermann assumed legal responsibility for all eight victims that was covered by the plea.

When they met he said there was a 'mutual respect.' 

'Throughout my course of my 30-year career, I have talked to Colombian drug lords, Colombian hitmen, MS 13 members, mafioso, blood members, child pornographers, it is about your level about professionalism. 

'You treat them like you want to be treated respectfully and they treat you the way you treat them with respect, so it was no different in this case,' he said.

Tierney said he believed Heuermann was being truthful when they spoke about the eight victims for which he admitted guilt.

'We had to establish each and every element of the crime charged against the seven, as well as the admission of guilt. As for Karen Vergata (his eighth victim)  I certainly believe that he was being truthful with regard to those things.'

When asked if Heuermann ever thought he would ever get caught, the DA said he did not know but what he did know was that he was 'smart, with an above average intelligence,  meticulous and manipulative.'

He spoke about Heuermann's double life and his facade and how he 'liked living in that space where he was not what he seemed to be'. 

'He was very good at  portraying himself to be who he thinks you want him to be - not only to his family, but also the people who knew him, worked with him, were friendly with him - did not ever ever think that he would be capable of what he's about to be convicted of,' he said. 

In the early days, long before he would find his victims from Craigslist, the 6-foot-4-inch, 270-pound would travel to designated areas in Brooklyn and New York where women walked the streets. 

He called him a 'prolific purveyor or consumer of escorts' and the vast majority he  did not murder.

However, the ones who became his victims had met him before as there was some type of familiarity and trust.

Tierney said Heuermann had met Amber Costello the day before. 'These women were working without the tightrope. They didn't have security,' he said in part.

He used a burner phone, which showed the crimes as pre- meditated.

'He was patronizing a lot of individuals and then at a certain point in time certain ones to murder,' he said.

'In the case of Megan Waterman, he purchased the phone June 4 made arrangements to meet her June 5, met her and unfortunately murdered her and shortly after that phone was never used again.'

The bail application he believed was created in 2000, and according to the DA showed a certain level of planning.

'As he progressed through the murders it became more considerable and more elaborate. Are we ever going to get the exact story?,' he asked. 'I don't know.' 

'People need to understand that each of these eight incidents there are two witnesses - one witness is unfortunately no longer with us - and then we have one witness, and than forensic evidence and other evidence.'

He did not want to go into the evidence out of respect for the families but did point out that Heuermann was familiar with some of the dumping sites and where he discarded the bodies.

Before the 61-year-old married architect was identified as a suspect, Tierney said he believed the perpetrator would be surveilling the news headlines through the media and the Internet and that he most likely got a 'thrill' from it. 

'As it turned out, he was monitoring the investigation,' Tierney said.

'He had gotten away with it for a long time. I think because he was somewhat sophisticated.

'I think he knew that there was always a chance that's why we wanted to be very secretive in what we were doing prior to making an arrest.'

During his election, Tierney remember the number of people who asked him about the Gilgo case

'People really kind of got in my face asking 'Are you going to do something about these women?' Help these families! Catch this guy!' he said.

When Tierney was elected and took office in 2021, he got to work and formed the Gilgo Beach Task Force. 

Part of their strategy, he explained was to make the suspect(s) think that their administration wasn't doing very much on the case.

He 'poked fun' at others who traveled to Gilgo to look for clues- and how the media just happened to be there. 

'Find a picture of me on that beach...you are not going to find it.'

Within weeks of forming the task force, Heuermann was identified as a person of interest in the case. 

'We wanted to rock the perpetrator to sleep and just told the victims families to be patient,' he said. 'When we did come forward he was in a jail cell.'

On the day Heuermann pleaded guilty there was a media frenzy outside the Suffolk County Criminal courthouse. 

Heuermann's estranged wife Asa Ellerup , 62, and daughter Victoria, 29,  emerged from the crowds with their attorney Robert Macedonio and therapist Alison Winter waiting to see it all play out in court.

Though wife and daughter knew months prior of the guilty confession that Heuermann had with each of them during a private jailhouse meeting.

After the hearing a press conference followed with the victims families and their attorney Gloria Allred by their side.

Tierney who hadn't been the chief attorney for Suffolk County during that time, addressed the inconsistencies in the case  - the politics, the misconduct, the fighting, the length of time it took.

Meeting with the victims families, he said many shared how their loved one was a daughter, a sister, a mother, a friend ... 'a real person'.

As a father of four children, Tierney and his team, showed up for them and during the press conference apologized to every victim's loved one by name.

'That is why one of the reasons why when we did the plea I wanted to apologize to each of the families who felt as though their loved ones got lost in it.'

Now that he is sending Heuermann to prison for life and to serve consecutive life sentences - his work is not over.

As part of the plea, he said Heuermann will be analyzed by the FBI's Behaviorial Unit Analysis (BUA).

'They can get real granular and explore what drives a person from being one way to actually committing murder', Tierney said. 

And, in Heuermann's case,  'their means, their methods, their motivations, how the police can prevent it, mitigate it, and detect it in the future.'

Tierney said Heuermann's DNA will be entered into CODIS - Combined DNA Index System - an electronic tool that compares DNA profiles with serial violent crimes to each other and to known offenders.

It is possible that this will reveal other victims beyond Suffolk County and even New York state. 

'We're going to continue to work on other cases, including trying to identify Asian Doe,' Tierney said referring to the still-unnamed victim whose remains were found near Gilgo Beach. 

'Hopefully, we'll figure out what happened with Asian Doe, and then we'll be able to go knocking on doors, whether it's the defendant's or someone else's.'

When asked if he believes Heuermann had other victims besides the eight women he has admitted to, Tierney would not say. 

'I believe a great many things, but it doesn't matter what I think. When you look at the history of this case everybody likes to say what they think,' he said. 

'We're investigating a great many cases, and if we get to the point where we know answers and we can say that one person or another did it, then we'll move on from there.'

On June 17, Heuermann will be sentenced to life in prison.