RIVERHEAD, N.Y. — The New York architect accused of murdering multiple women and leaving their corpses scattered along the Long Island coast kept a “blueprint” of his crimes on his computer, prosecutors revealed Thursday as they brought charges against Rex Heuermann in two more killings.

Heuermann, 60, appeared before a judge to be arraigned in the deaths of Jessica Taylor and Sandra Costilla, two young women who were long suspected of being the victims of men preying on sex workers. He had previously been charged with murdering four other women in a string of deaths known as the Gilgo Beach serial killings.

Taylor disappeared in 2003. Costilla was killed 30 years ago, in 1993, and her inclusion in the case indicates that prosecutors now believe Heuermann was killing women for much longer than previously thought.

The new charges came after recent police searches of Heuermann’s Massapequa Park home and a wooded area on Long Island tied to the investigation.

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Forensic Evidence and ‘Blueprint’ Document

In a court filing, prosecutors said they were able to use new forensic testing methods to match hairs found on or near the vicinity of both victims to a DNA profile that is a likely match to Heuermann. Additionally, prosecutors say they recovered a file on a hard drive in his basement used to “methodically blueprint” his killings.

The all-caps document features a series of checklists with tasks to complete before, during and after the killings, as well as practical lessons for “next time.” Among the dozens of entries written are reminders to clean the bodies and destroy evidence, to “get sleep before hunt” and to “have story set.”

One section, titled “things to remember,” appears to highlight lessons from previous killings, prosecutors said, such as using heavier rope and limiting noise in order to maximize “play time.” A “body prep” checklist includes, among other items, a note to “remove head and hands.”

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Additional Victims and Future Investigations

Prosecutors believe that entry may connect Heuermann to yet another victim, Valerie Mack, whose partial skeletal remains were discovered near the body of Taylor after her disappearance in 2000.

Heuermann has not been charged in the death of Mack. But asked during a news conference after Thursday’s hearing if he was a suspect, District Attorney Ray Tierney replied, “That’s fair to say.”

Tierney also acknowledged that the “blueprint” document, which Heuermann had attempted to delete, was a “significant impetus” for the renewed search across Long Island in recent weeks, as it was recovered in March from the more than 350 electronic devices seized from the suspect’s home.

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Heuermann pleaded not guilty to killing Taylor and Costilla during the hearing and was ordered held without bail. His lawyer, Michael Brown, said outside court that Heuermann is “obviously in a bad place in terms of the new charges.”

Afterward, Tierney said the additional charges provide “some small measure of closure” for the victims’ families.