Sara Karnes was sitting in her Times Square hotel room when she says she received a cell phone call from a blocked number.

She said the voice on the other line, an unknown male caller who was calm and matter of fact, revealed personal details about her friend, Maureen Brainard-Barnes, who vanished days earlier in July 2007 while working as an escort in New York City.

"'I just saw her,'" Karnes recalls the man saying. "'She's at a whorehouse in Queens.'"

Exactly 16 years after the chilling phone call, Karnes said she believes that mystery voice was that of Rex Heuermann, a married father of two and New York architect who is charged in the murders of three other women. He is the prime suspect in the death of 25-year-old Brainard-Barnes, according to investigators.

Heuermann has pleaded not guilty to all charges.

"That was him," Karnes said of the caller in an exclusive "20/20" interview following Heuermann's arrest on July 13.

Heuermann, 59, is charged in the killings of Melissa Barthelemy, 24, Megan Waterman, 22, and Amber Lynn Costello, 27, who were working as escorts on Craigslist at the time they disappeared.

The badly decomposed remains of those women, as well as those of Brainard-Barnes, were found in 2010 near Gilgo Beach on New York's Long Island in what police described as the work of a serial killer.

Karnes, who reported the phone call to police at the time, said she made the connection after hearing Heuermann's voice in a now-viral interview on YouTube about his work as an architect.

"I remember rewinding it and playing it over and over again," Karnes said.

Watch the all-new "20/20" episode, "The Long Island Serial Killer," airing Friday, Sept. 8, at 9 p.m. ET and streaming the next day on Hulu.

The investigation into the Long Island serial killer focused on cell phone use from the beginning, according to investigators. The victims, known as the "Gilgo Four," posted advertisements on Craigslist and then used their own cell phones to connect with clients.

The police claim the killer, meanwhile, used separate burner phones to contact each victim – and, in doing so, gave them their first clue.

"We had cell site hits in two areas in Massapequa Park and also in midtown Manhattan," Suffolk County Police Commissioner Rodney Harrison told ABC News.

The authorities claim the killer also used the cell phone of one of his victims, Melissa Barthelemy, to make a series of taunting phone calls to her family in Buffalo, N.Y., in the summer of 2009. Those calls allegedly pinged from towers near New York's Penn Station and other areas of Manhattan.

"I'm watching your sister rot," the killer said to Barthelemy's younger sister, according to multiple sources with knowledge of the phone calls.

It wasn't until 2021 that investigators, including the FBI, said they were able to use advanced phone analytics to hone in on an area of Massapequa Park – some 17 miles from Gilgo Beach -- where they believed the killer likely lived.

"There was a little bit of what we called a box, where our potential suspect was making these phone calls to our sex workers," Harrison said.

The next breakthrough in the case came in the form of an old clue that police revisited in March 2022.

A witness reported seeing a dark green Chevy Avalanche parked outside the home of Amber Lynn Costello in Babylon, N.Y., the day before she disappeared in September 2010.

The witness also told police that Costello met with a client that same day, someone the witness described as "a white male, approximately 6'4" to 6'6" in height, in his mid-40s, with 'dark bushy hair,' and 'big oval style 1970's type eyeglasses,'" according to court documents in the case.

Last year, investigators were able to determine that Heuermann – who lived in Massapequa Park and fit that physical description – owned a Chevy Avalanche in 2010.

"We were able to capture -- were able to seize that Chevy Avalanche pursuant to a search warrant and we're certainly going to analyze that," Suffolk County District Attorney Ray Tierney said at a press conference the afternoon after Heuermann's arrest.

Once authorities had established Heuermann as a suspect, they conducted surveillance on his daily life, issuing 300 subpoenas and search warrants to collect potential evidence.

In January 2023, an undercover team watched from security cameras as Heuermann allegedly tossed a pizza box into a trash can outside of his midtown Manhattan office.

The team retrieved the box, which contained a partial piece of pizza crust that was later tested for DNA.

It proved to be a "99.96%" match to hair found on one of the victims at Gilgo Beach, according to authorities.

Michael Brown, Heuermann's attorney, scoffed at reporters who asked about the alleged physical evidence following Heuermann's court appearance on July 14 in Riverhead, N.Y.

"I didn't see any evidence, did you?" Brown said, claiming the government's case against his client is "very circumstantial."

The hunt for the killer began with the disappearance of Shannan Gilbert, a 23-year-old escort from Jersey City, N.J., who disappeared on May 1, 2010, from Oak Beach, a gated community on the South Shore of Long Island.

The police search to find Gilbert led to the discovery of four sets of human remains dumped in thick bramble along Ocean Parkway at Gilgo Beach in December 2010.

The women – three of them wrapped in camouflage burlap – were all bound in a similar fashion and placed approximately 500 feet apart from one another.

Gilbert's body was not among the first four found in Gilgo Beach. The search, consisting of K-9 and marine units, expanded to the surrounding area with the assistance of the FBI.

Gilbert’s skeletal remains were found in a marsh in Oak Beach in December 2011. The Suffolk County Medical Examiner ruled her cause of death as “undetermined” and police said they believe her death was an “accident” — with some investigators theorizing Gilbert drowned in the marsh.

The Gilbert family, however, maintains that foul play was to blame for her death.

In 2015, the family retained forensic pathologist, Dr. Michael Baden, to conduct an independent autopsy. His report concluded that Gilbert’s death was a likely homicide.

Six more sets of remains were found along Ocean Parkway : the skull, hands and forearm of Jessica Taylor, 20 (whose torso was found in Manorville in 2003); skull, foot and hands of Valerie Mack, 24, (torso found in Manorville in 2000); skeletal remains of an unidentified male dressed in women's clothing; unidentified female toddler and the bones of a woman believed to be her mother; and a skull linked to a torso found in 1996 on Fire Island, identified in August 2023 as 34-year-old Karen Vergata.

To this day, authorities do not know if the deaths are the work of one or more killers.

"The investigation into the remaining bodies is still continuing," Tierney said.

While Rex Heuerman's attorney declined to comment when asked if his client was involved in the deaths of Maureen Brainard-Barnes and others found along Ocean Parkway, he tells ABC News, in part, "In a court of law what’s required is evidence and proof beyond a reasonable doubt… We look forward to defending this case in a court of law."

For Sara Karnes, the pain of losing her friend is as raw today as it was nearly two decades ago.

"Every time I talk about it, it doesn't get any easier," Karnes said. "She had these bright blue eyes. She was very outgoing, very outspoken."