With Rex Heuermann charged with 2 more Long Island murders, investigators trying to find how deep the case goes

A deleted document that investigators recovered from a laptop may be the key to establishing intent in the case against Rex Heuermann, who has been charged with the murders of six women over several decades, Suffolk County District Attorney Raymond Tierney told ABC News on Friday.

That document, which investigators described as a "planning document," was discovered in a veritable mountain of electronic devices and files recovered during the investigation, Tierney said. It was found on March 7, 2024, on a laptop's unallocated space, meaning it had been deleted by a user, the district attorney said.

Prosecutors allege it was used by Heuermann to "methodically blueprint and plan out his kills with excruciating detail."

"I think it, first and foremost, evinces a certain intent, and that intent is to hunt and capture and kill females," Tierney told ABC News in an interview, some of which aired on "Good Morning America."

His office was "excited" about the document, because it "establishes intent, and certainly that's our allegation -- and that's what we seek to prove," Tierney said.

Investigators on the case said they found the document as they worked through a wealth of tech items recovered during the investigation. They so far have recovered 27 computers and 58 internal hard drives, along with another 22 external drives. They've collected 15 types of cameras. And there are dozens of memory cards and USB devices and SIM cards. Hundreds of CDs and VHS cassettes were also logged by investigators.

That document was discovered almost a year after the initial charges were filed in July 2023 against Heuermann, a Long Island architect who has pleaded not guilty to all the charges against him.

Heuermann was charged Thursday with the 2003 murder of Jessica Taylor, whose remains were found on Long Island at Gilgo Beach and in Manorville, and the 1993 murder of Sandra Costilla, whose remains were found in North Sea, Long Island, in 1993, according to court documents.

These developments expand the timeline in the Gilgo Beach case by a full decade.

The charges brought against Heuermann allege that he acted alone, according to court documents. Prosecutors have said in charging documents that they've pieced together a timeline of the murders using digital calendars and notes from some of the devices they've collected. Heuermann's estranged wife, Asa Ellerup, and their children were away from their Massapequa Park home during the commission of the crimes, investigators allege.

"With regard to the family, it's very clear that during the commission of the six crimes … they were either outside of the state or outside of the country," Tierney told ABC News.

Investigators have spent months essentially going "back in time" through the "tremendous" amount of evidence in the Heuermann case, Tierney said.

They've also during the same period been going through cold-case killings and other open investigations, he said. Several other bodies were discovered over the years on Gilgo Beach, near the brambles where the bodies of four of Heuermann's alleged victims were discovered.

"We're going to continue to look at the cases," Tierney said on "GMA" on Friday. "We've got more bodies on Gilgo Beach and we have more bodies off of the beach as well, which has resulted in these two additional charges. We'll continue to work."

ABC News asked Tierney whether his office had thought the allegations against Heuermann would stretch as far back as 1993.

"In the beginning, we knew that this was going to be a sprawling, significant case," Tierney said. "And we really had no preconceptions. I think the better way to answer that: Am I surprised? No."