Suspected Long Island serial killer Rex Heuermann is under new scrutiny as investigators have returned to his home nearly a year after his arrest on multiple murder charges in the Gilgo Beach case, which perplexed authorities for over a decade until last year.

After a New York task force placed Heuermann under arrest last year, he became a potential cold case person of interest in other states where he has ties – New Jersey, Nevada, South Carolina and Connecticut.

"If they attach one of those cases to him, this turns federal and this whole thing goes belly up," said Joseph Giacalone, a former cold case investigator with the NYPD and an adjunct professor at John Jay College of Criminal Justice. "We know the FBI is involved, and the FBI plays the long game – and they're always working."

If the case does go federal, prosecutors can seek capital punishment.

REX HEUERMANN'S FAMILY KEPT GRUESOME PIECE OF EVIDENCE, SOURCE SAYS

Rex Heuermann headed to court

Alleged Gilgo Beach serial killer Rex A. Heuermann, right, appears in Judge Tim Mazzei’s courtroom next to his attorney Michael Brown at the Suffolk County Court in Riverhead on April 17, 2024. (James Carbone/Newsday)

"The feds can then bring death penalty charges into it, even in New York state, and that could get him to talk, and try to close out some of these other cases that he may or may not be involved with," Giacalone said.

It's rare for the federal government to seek the death penalty, but it does happen in cases of extreme violence, such as in the Boston Marathon bombing, and prosecutors can use it as leverage to get a confession.

"That’s the ace in the hole for having the feds take over," he told Fox News Digital. "They can get somebody to talk and save their own miserable life."

SUSPECTED SERIAL KILLER REX HEUERMANN'S HOME SEARCHED AGAIN

GALLERY: Police return to home of suspected serial killer Rex Heuermann to take more evidence

It's a long shot that Heuermann, if convicted, would actually face capital punishment, however.

"By offering him a waiver of the death penalty it may persuade him to lock it down for the feds," said David Gelman, a New Jersey defense attorney and former prosecutor. "But his lawyers, I’m sure, realize that the death penalty is like a unicorn – it almost never happens."

Much of the evidence police have been seen taking out of Heuermann's house this week has been hidden from view in boxes or bags.

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Gilgo Beach murders map

The locations where eight of 10 bodies were found near Gilgo Beach since December 2010 are seen in this Suffolk County Police handout image. (Reuters/Courtesy of Suffolk County Police)

Investigators have declined to comment on the new search, but they are believed to be looking for evidence in another cold case and not one of the four that Heuermann already faces charges in, Giacalone said. The evidence in those four cases is already "overwhelming," he added.

Heuermann is accused of leaving his wife's hair on materials found on the victims. Suffolk County District Attorney Ray Tierney said publicly shortly after his arrest that investigators believe DNA from Asa Ellerup, Heuermann's wife, was transferred to the crime scene by the suspect and that she is not accused of helping in the slayings.

She filed for divorce shortly after her husband's arrest but has visited him in jail and said she will withhold judgment until after his trial.

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Portraits of the Gilgo Four victims inset over a wide shot of the marsh behind Gilgo Beach where they were found

The "Gilgo Four," clockwise from top left, Maureen Brainard-Barnes, Melissa Barthelemy, Megan Waterman and Amber Costello. The background shows a wooden cross in the marsh next to Gilgo Beach, New York, where their remains were found in the brush just yards from Ocean Parkway. (Suffolk County Police Department/Mega for Fox News Digital)

Giacalone said he may have similarly contaminated another crime scene with pet hair, and images show police removing a dog bed from Heuermann's house.

"Why else take the dog bed unless you have a dog hair or an animal hair to match it up to?" he asked.

Heuermann has been charged with the murders of four women whose remains were all discovered near one another next to Gilgo Beach along Ocean Parkway, about 40 miles east of New York City.

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Police arrested him in July in connection with the murders of Melissa Barthelemy, 24; Megan Waterman, 22, and Amber Costello, 27. They later added more charges for the death of Maureen Brainard-Barnes, 25. 

But investigators found 11 bodies total across a span of several miles along the highway after another woman, Shannan Gilbert, vanished after placing a panicked 911 call in 2010. Police later deemed her death an accident, but the attorney for her family believes there was foul play involved.

Closeup portrait Photos of Jessica Taylor and Valerie Mack.

Jessica Taylor, left, and Valerie Mack, right, were both murdered and dismembered. Suffolk County police discovered partial remains of each victim in both Manorville, New York, and along a stretch of Ocean Parkway near Gilgo Beach. (Suffolk County Police Department)

Two of the other victims, Jessica Taylor and Valerie Mack, were both dismembered and concealed in separate locations.

Police first discovered each of their partial remains in Manorville in 2000 and 2003. Additional remains of both victims were uncovered in the search for Gilbert in 2011.

Shannan Gilbert missing woman Long Island

Shannan Gilbert’s remains were found near Oak Beach, New York, on Dec. 13, 2011.  (The family of Shannan Gilbert)

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In April, police from multiple agencies returned to a stretch of woods in Manorville with K-9 units. It's unclear what they found, and they have declined to comment on the active investigation.