As the tangled investigation into the Gilgo Beach serial killings suspect continues, a police official described to CNN the tense moments leading up to his arrest, including fears that the man might flee the country if he found out that prosecutors were preparing charges in the case.

"It's a very good thing that we got this animal off the streets," Suffolk County Police Commissioner Rodney Harrison said on CNN's "OutFront" Monday night.

Authorities have said the suspect, Rex Heuermann, obsessively followed the case of the "Gilgo Four," a group of four women whose remains were uncovered more than a decade ago near Long Island's Gilgo Beach. His internet history revealed at least 200 searches for information about the investigation, as well as compulsive searches for photos of the victims and their families, according to prosecutors.

As prosecutors brought the case to a grand jury last week, authorities were concerned that Heuermann could find out about the proceedings and try to evade arrest, possibly by leaving the country, Harrison said.

Heuermann, an architect and father of two, was under 24-hour surveillance before he was arrested Thursday and charged with murder in three of the Gilgo Four slayings, according to the police commissioner. He has pleaded not guilty to the killings of Melissa Barthelemy, Megan Waterman and Amber Costello and remains the prime suspect in the killing of the fourth victim, Maureen Brainard-Barnes, according to court documents.

Though he has yet to be charged in the killing of Brainard-Barnes, Suffolk County District Attorney Ray Tierney told CNN he is "confident" prosecutors will be able to bring a charge in the case.

As Heuermann was being processed at the jail following his arrest, he had just one question, a source said: "Is it in the news?"

Despite an arrest in the case, investigators say their work is far from over.

Authorities are "actively investigating" whether Heuermann may have had more alleged victims, Deputy Police Commissioner Anthony Carter said Monday on "CNN Tonight." Asked Sunday whether investigators believed the suspect may have set his sights on a future victim, Carter said it was "always a concern."

A search of Heuermann's Massapequa Park, Long Island, home revealed a hoard of between 200 and 300 firearms that had been stashed in a walled-off vault which was locked behind a metal door, a source with knowledge of the case told CNN.

As investigators scour Heuermann's home, office and a storage unit he kept, they are looking for any "out of place" items, a source familiar with the case told CNN. The source said one such object includes a doll found in the house but not in the children's rooms.

Though the searches are primarily focused on gathering forensics, authorities are also keeping an eye out for items that could be hidden or surreptitiously stored souvenirs from the killings, the other source with knowledge of the case said.

Heuermann's family was shocked when investigators told them about the allegations against him, Harrison said.

"They were disgusted, they were embarrassed," the commissioner said. "So, if you ask me, I don't believe they knew about this double life that Heuermann was living."

Source: Authorities are considering possibility of more killings

As authorities painstakingly search for evidence, they are operating under the assumption that the suspect may have continued killing after the Gilgo Four were discovered in 2010, which may lead to a prolonged investigation, a source familiar with the case said.

Investigators may also have to undertake the time-consuming task of reviewing pieces of evidence with victims' family members and friends in order to determine if they are souvenirs from the killings, another source said.

"This investigation is still in its infancy," Carter, the deputy police commissioner, said Monday. "We have a long way to go. We have a lot of evidence to process, a lot more witnesses to interview and tips are still coming in."

Heuermann was first identified as a potential suspect in early 2022, shortly after a multi-agency task force was convened to examine the cold cases of nearly a dozen human remains found along Long Island's South Shore between 2010 and 2011, including the Gilgo Four.

Investigators were able to connect Heuermann to at least three of the killings using cell phone data, credit card bills and a sample of DNA taken from a pizza crust the suspect discarded in a Manhattan trash can, according to prosecutors.

Though prosecutors have said Heuermann is the prime suspect in the fourth killing of Brainard-Barnes, he has yet to be charged in the case. Police commissioner Harrison said Monday that a charge in the case may "take a little time" because a hair follicle they collected as evidence was damaged and needs to undergo DNA testing.

Meantime, law enforcement is sifting through a "flood" of tips and evidence that have come in following Heuermann's arrest, Tierney, the district attorney, said Monday.

"We have a description of the last individual who saw the final victim, Amber Costello, alive that matches that of this defendant, who has a rather unique physical appearance," Tierney added. "We have phone evidence, and we also have that DNA evidence."

Heuermann's DNA was found with one of the bodies, and his wife's DNA was found on two bodies, Tierney said.

"With regard to that DNA and those hairs, it's either transfer -- in other words, one person comes into contact with a second person who then comes into contact with a third person -- or, they were at a location where, you know, the family member occupied, whether it's a car or a house," Tierney said.

Among the hundreds of firearms found hidden in Heuermann's home were pistols, revolvers and semi-automatic rifles, a source said. The stash was far more extensive than authorities expected. They were only aware of 92 firearms the suspect had registered in the state.

Victims were found bound and buried in a similar way, police say

The remains of the Gilgo Four were found within three days of each other along the same quarter-mile stretch of Ocean Parkway -- each wrapped in camouflaged burlap, according to authorities.

"They were buried in a similar fashion, in a similar location, in a similar way. All the women were petite. They all did the same thing for a living. They all advertised the same way. Immediately there were similarities with regard to the crime scenes," Tierney said last week.

Three of the women -- Costello, Waterman and Brainard-Barnes -- worked as escorts and Barthelemy was a sex worker, according to police. All four women advertised their work on Craigslist.

Barthelemy was last seen at her apartment on July 12, 2009, when she told a friend she was going to see a man, according to a Suffolk County website about the killings.

Brainard-Barnes was last seen in early June 2007 in New York City and Costello was last seen leaving her North Babylon home in early September 2010, according to police. Waterman was last seen in early June 2010 at a Holiday Inn Express in Hauppauge.

"These victims were mothers, daughters and people in the community," Carter said Sunday. "Nobody deserves what happened to them. ... To bring their families and bring the victims themselves justice -- it is long overdue."