After 13 years of dead ends and blown leads, the Gilgo Beach murder investigation finally turned on pizza crusts that Rex Heuermann had tossed in a trash can in Midtown Manhattan.

It was a jackpot for investigators who had watched Mr. Heuermann for months.

“Pizza crust is like a sponge — it allowed the saliva to seep into the dough,” Ray Tierney, the Suffolk County district attorney, said in a recent interview.

The sample gave investigators the genetic match that helped connect Mr. Heuermann to four bodies found in 2010 on Long Island, and his arrest followed in July, Mr. Tierney said. When Mr. Heuermann’s trial begins, possibly this year, the DNA evidence will underpin the charges that he murdered women he had hired as escorts and left their bodies wrapped in burlap along a desolate oceanfront parkway.

Investigators say the DNA profile obtained from a male hair found on the burlap used to wrap one of the four victims found in 2010, Megan Waterman, corresponds to the pizza slice sample from Mr. Heuermann.

The authorities have charged Mr. Heuermann with three of the killings, but refrained from a fourth charge because genetic test results were incomplete in July. Mr. Heuermann has pleaded not guilty and is being held without bail at a Suffolk County jail.

Prosecutors have said Mr. Heuermann will likely be charged this month in connection with the fourth body, and on Monday morning they announced he would appear in court in Riverhead on Tuesday, with prosecutors holding a news conference afterward.