Authorities say they've unearthed chilling evidence in the case of the Long Island serial killer—including his alarming search history.

Days after suspect Rex Heuermann was arrested and charged in connection to the murders of three women found in Gilgo Beach over a decade ago, the prosecuting attorney on the case has given insight into their investigation.

According to authorities, once Heuermann was identified as a suspect, they say that investigators were able to trace his burner phones, which led to the discovery of additional burner phones, fake email accounts and false identities he had used in the process of "gathering a massive amount of digital evidence and trace evidence."

"We saw all this, really sort of concerning searches that he was undergoing," Suffolk County District Attorney Ray Tierney told People July 19. "In a 14-month period, over 200 times, he's searching for information about the Gilgo investigation. He's trying to figure out what we're up to."

According to NBC New York, prosecutors said questions included in his search history included, "Why could law enforcement not trace the calls made by the long island serial killer" and "Why hasn't the long island serial killer been caught." Additionally, investigators also allege they found hundreds of internet searches about sexual abuse toward women and child pornography, as well as searches for victims and their families.

"He was obsessively looking at the victims," Tierney noted. "But he's also looking at the victim's siblings."

Heuermann was also seemingly captivated by other serial killers, per authorities, with his online history featuring searches for "11 currently active serial killers," and "8 Terrifying Active Serial Killers (We Can't Find)."

According to Tierney, Heuermann, a 59-year-old man from Massapequa Park, was "pretty surprised" when he was arrested July 13.

"I think he lived this double life, and he used the anonymity of phones and computers to shield himself from the rest of society," the district attorney said. "Unfortunately for him—and fortunately for the rest of us—he wasn't successful."

Upon his arrest, Heuermann was charged with three counts of first-degree murder and three counts of second-degree murder in the deaths of Melissa Barthelemy, 24; Megan Waterman, 22; and Amber Lynn Costello, 27. He pleaded not guilty to all counts at an arraignment on July 14, per his defense attorney Michael J. Brown.

"There is nothing about Mr. Heuermann that would suggest that he is involved in these incidents," Brown said in a July 14 statement to E! News. "And while the government has decided to focus on him despite more significant and stronger leads, we are looking forward to defending him in a court of law before a fair and impartial jury of his peers."

According to NBC News, he is also suspected in the disappearance and death of a fourth woman, Maureen Brainard-Barnes, 25, whose remains were also found near Gilgo Beach.
 
The women were among the remains of 11 people who were discovered after the 2010 disappearance of Shannan Gilbert kickstarted an investigation. (Her remains were found by police on Oak Beach in December 2011.)

Suffolk County Police Commissioner Rodney Harrison previously described Heuermann as "a demon that walks amongst us, a predator that ruined families." 

"However, even with this arrest, we're not done," Harrison said during a July 14 press conference. "There's more work to do in the investigation in regards to the other victims of the Gilgo Beach bodies that were discovered."

(E! and NBC News are part of the NBCUniversal family.)