Accused Gilgo Beach killer Rex Heuermann is charged with two more murders. Investigators are trying to determine how deep the case goes.

LONG ISLAND — A deleted document that investigators recovered from a laptop could be key to proving intent in the case against Gilgo Beach serial killer Rex Heuermann, who is accused of murdering six women over several decades, Suffolk County District Attorney Raymond Tierney told ABC News on Friday.

That document, which investigators called a “planning document,” was discovered in a veritable mountain of electronic devices and files seized during the investigation, Tierney said. It was found on March 7, 2024, in the unallocated space of a laptop, meaning it was deleted by a user, the district attorney said.

The prosecution claims that Heuermann used the material to “plan his murders methodically and with the utmost detail.”

“I think it primarily demonstrates some intent, and that intent is to hunt, capture and kill females,” Tierney said in an interview with ABC News, part of which aired on “Good Morning America.”

His office was “excited” about the document because it “demonstrates intent, and that is certainly our claim – and that is what we are trying to prove,” Tierney said.

Investigators on the case said they found the document while going through a plethora of technical items seized during the investigation. So far, they have seized 27 computers and 58 internal hard drives, as well as 22 other external drives. They have collected 15 types of cameras. And there are dozens of memory cards, USB devices and SIM cards. Hundreds of CDs and VHS tapes have also been logged by investigators.

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Suffolk County District Attorney Raymond Tierney discusses the investigation into accused serial killer Rex Heuermann.

This document was discovered nearly a year after the first indictment was filed in July 2023 against Heuermann, a Long Island architect who has pleaded not guilty to all charges against him.

Heuermann was charged Thursday with the 2003 murder of Jessica Taylor, whose remains were found on Long Island in Gilgo Beach and in Manorville. He was also charged with the 1993 murder of Sandra Costilla, whose remains were found in North Sea, Long Island, in 1993, court records show. These developments greatly expand the time frame in the Gilgo Beach case, as the first murder Heuermann is now accused of committing now occurred in 1993.

The charges against Heuermann allege he acted alone, court records say. Prosecutors said in charging documents that they constructed a chronology of the murders using digital calendars and notes from some of the devices they collected. Heuermann's estranged wife, Asa Ellerup, and their children were not at their Massapequa Park home during the crime, investigators claim.

SEE ALSO: Public prosecutor: “Blueprint” of the murders leads to charges against Heuermann in connection with the deaths of two other women

“As far as the family is concerned, it is quite clear that they were either out of state or out of the country at the time the six crimes were committed,” Tierney told ABC News.

Investigators essentially spent months “going back in time” and going through the “massive” amount of evidence in the Heuermann case, Tierney said.

During the same period, they also worked on unsolved murder cases and other open investigations, he said. Over the years, several more bodies were discovered on Gilgo Beach, near the bramble bush where the bodies of four of Heuermann's alleged victims were found.

“We're going to continue to pursue the cases,” Tierney said on Friday on “GMA.” “We have additional bodies on Gilgo Beach and also off the beach, which has led to these two additional charges. We're going to continue to work on that.”

RELATED: Gilgo Beach murder investigation expands to include human remains

ABC News asked Tierney if his office had expected the allegations against Heuermann to date back to 1993.

“We knew at the beginning that this was going to be a big, big case,” Tierney said. “And we really had no preconceptions. I think the better answer to that is: Am I surprised? No.”

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