Local authorities arrested the alleged Long Island serial killer earlier than expected after discovering the prime suspect of the Gilgo Beach murders was keeping tabs on the investigation online, according to the district attorney involved in the case.

Suffolk County District Attorney Ray Tierney told Fox News on Monday that Rex Heuermann did not anticipate being taken into police custody last week, despite allegedly following the case’s progress.

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“I think he was extremely surprised," Tierney said. “I think we utilized the grand jury because we wanted to maintain investigative secrecy. We were concerned about that all along.”

Tierney added law enforcement and prosecutors were “pleased” they were able to “maintain that secrecy.”

To follow the investigation, Heuermann used Google to search related terms, according to Tierney.

“He was asking questions such as … ‘What is the phone evidence in the Gilgo case? How … come they can't trace the phones in the Google case?’ So he's asking specific questions with regard to our investigative means,” the New York district attorney said.

“He was also making searches of the victims, looking at pictures of the victims online, and he was running searches of the victims' siblings and, in one instance, admitted that he was the killer,” added Tierney. “He had some disparaging things to say about the sister and said he was going to come find her.”

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Suffolk County police caught Heuermann last Thursday night at his home after more than a decade of investigation, and news of the arrest was made public Friday morning. The alleged serial killer was charged with three of the 11 deaths related to the case, charges to which he pleaded not guilty later Friday.

The criminal investigation into Heuermann is still ongoing as law enforcement reportedly seeks to connect him to the murders of eight more victims.