Sarah Butler was nervous.

The 20-year-old college student had borrowed the keys to her mother’s van, explaining that she wanted to meet up with a friend while home in Montclair, New Jersey, during the Thanksgiving break.

What she didn’t mention, though, was that the “friend” was an online acquaintance offering to pay her $500 for sex. She had backed out of meeting him in person before, authorities say, but this time, she planned to go through with it.

“You’re not a serial killer, right?” she messaged him before leaving the house. Khalil Wheeler-Weaver was exactly that, prosecutors said on Thursday.

The 23-year-old is accused of murdering three women and attempting to kill a fourth, and authorities say that Butler was his final victim.

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Before the two met up on 22 November 2016, he searched the internet for information about date-rape drugs and homemade poison, according to the North Jersey Record.

Ten days later, Butler’s body was found in a nature reserve, covered with leaves and detritus. She had been strangled.

According to prosecutors, Mr Wheeler-Weaver targeted young black women who turned to sex work while coping with mental health issues or homelessness. His thinking, authorities say, was that no one would notice if they disappeared.

“They were viewed as somehow less than human, less valuable,” Essex County assistant prosecutor Adam Wells said in October, according to local media.

Butler was a longtime lifeguard at the local YMCA and second-year student at New Jersey City University known for her talents as a dancer.

After she disappeared, her friends and family launched a sting operation that led authorities to identify Mr Wheeler-Weaver as the prime suspect in a months-long crime spree that took place in abandoned houses and budget motels across northern New Jersey.

Mr Wheeler-Weaver has admitted being with each victim shortly before they disappeared but denies responsibility for their deaths, NJ.com reported.

His attorneys contend that the victims “put themselves in vulnerable positions”, and point out that Mr Wheeler-Weaver cooperated with investigators, which they argue “is not the conduct of a guilty individual”.

“He told police where he had last seen them and that they were alive and safe,” his public defender, Deirdre McMahon, said in court last month, according to NJ.com. “What happened to them afterward is not at the hands of Mr Wheeler-Weaver.”

With a carefully trimmed beard and rectangular glasses that give him the affect of a graduate student, Mr Wheeler-Weaver “doesn’t look like someone who would’ve done something like this”, Mr Wells said during a court hearing last month.

After the first of his alleged victims, 19-year-old Robin West, was found dead in September 2016, police described him as a “calm” and “helpful” witness who gladly answered their questions.

West, a Philadelphia native who struggled with mental health issues and left home at a young age, was days away from celebrating her 20th birthday when she vanished, alarming her family when she failed to respond to their messages about party plans.

On 1 September 2016, authorities got a call about a fire at an abandoned house in Orange, New Jersey. Inside, they found Ms West’s body, which was so badly scorched that it took nearly two weeks to identify her through her dental records.

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When questioned by police, Mr Wheeler-Weaver said that he had taken West out to eat on the day that she disappeared, NJ.com reported.

Then, he claimed, he dropped her off at a different abandoned house, roughly two blocks away from where the fire started. While investigators were puzzling over the crime, another woman disappeared.

Joanne Brown, who had been grappling with homelessness and mental health issues, was seen getting into Mr Wheeler-Weaver’s car before she was reported missing in October 2016.

Less than two months later, a work crew found the 33-year-old’s remains inside yet another vacant house in Orange. Her nose and mouth were covered with tape, and a jacket was tied around her neck. She had been strangled.

By the time Brown’s body was found, another woman had come forward to describe a terrifying encounter with Mr Wheeler-Weaver. The woman, who was 34 at the time and is identified only as “T.T.” in court documents, had turned to sex work after becoming homeless.

By 15 November, 2016, though, she was several months pregnant, and looking for another way to make money. She told authorities that she agreed to have sex with Mr Wheeler-Weaver, but actually planned on tricking him and taking his cash.

The two met at a cheap motel in Elizabeth, New Jersey, then left in Mr Wheeler-Weaver’s car. In court last month, the woman testified that Mr Wheeler-Weaver, clad in a ski mask, handcuffed her and covered her mouth with duct tape before raping her in the back seat and almost strangling her to death.

She repeatedly lost consciousness, she said, but woke just long enough to formulate an escape plan.

She convinced Mr Wheeler-Weaver to take her back to the motel, where she had left her cellphone, then locked him out of the room while she dialled 911. But police who responded to the call were more interested in figuring out if she was a prostitute, she said in court last month.

According to NJ.com, an officer from the Elizabeth Police Department who was called to testify said that he initially didn’t believe the woman when she called to report a kidnapping because she waited an hour to do so.

Months would pass before Mr Wheeler-Weaver was arrested on attempted murder charges.

On 22 November 2016, exactly one week after “T.T.” told authorities about her frightening encounter, he met up with Butler in Orange. When the college sophomore didn’t return home the next morning and wasn’t answering her phone, her family called police.

On 1 December 2016, Butler’s body was found in the nature preserve, confirming their worst fears.

It took just five days for police to arrest Mr Wheeler-Weaver and charge him in connection with her murder.

Prosecutors revealed in court last month that Ms Butler’s parents, sister and friends had taken matters into their own hands, logging into the 20-year-old’s social media accounts to see whom she had been talking with before she disappeared.

(Google Maps (Google Maps)

On a controversial social networking app called Tagged. which has been linked to sex crimes and criticised for taking a lax approach to child pornography, they found her messages with Mr Wheeler-Weaver.

Butler’s friends and family created a fake profile on the site, using the promise of sex to lure Mr Wheeler-Weaver into an in-person meetup. When he arrived, prosecutors say, he found himself face-to-face with the police.

“Sarah’s friends and family are the heroes of this case,” Mr Wells said last month, according to local media.

Authorities found that Mr Wheeler-Weaver had conducted a slew of disturbing online searches before meeting Butler, including “How to make homemade poisons to kill humans” and “What chemical could you put on a rag and hold to someone’s face to make them go to sleep immediately.”

He had reportedly researched how household chemicals like bleach and ammonia could be used to kill someone, and if it was possible to erase your phone data and avoid being tracked, according to the North Jersey Record.

Another search, for a practice test for the police entrance exam, suggested that Mr Wheeler-Weaver, who had been working as a grocery store security guard, hoped to become a police officer.

That prospect became increasingly unlikely as Mr Wheeler-Weaver faced a mounting series of criminal charges, linking him to the two other women’s deaths.

His phone records contradicted his story about the last time he had seen West, according to prosecutors, who contend that Mr Wheeler-Weaver was at the same house where her body was found shortly before it went up in flames.

After driving off, he reportedly returned to the neighbourhood and watched as firefighters tried to subdue the blaze.

Mobile phone records also showed that Mr Wheeler-Weaver was the last person to call Brown before she disappeared. Authorities say that he picked her up and brought her to the vacant house where her body was later found, and spent about an hour there with her before leaving alone.

In February 2017, Mr Wheeler-Weaver was indicted on three counts of murder, and one count of attempted murder stemming from the near-fatal attack in Elizabeth.

He has also been charged with desecration of human remains, aggravated arson, aggravated sexual assault and kidnapping. His trial in the Superior Court of Essex County is slated to continue this week.

To many of the victims’ families, the gruesome details that authorities have revealed so far are baffling. Mr Wheeler-Weaver was shockingly young at the time when the attacks allegedly occurred, and hardly fits the profile of a serial killer.

“I need to know what happened to him, what caused him to snap like this,” Leroy West, the father of Robin West, told WHYY in 2017.

“I’ve been face-to-face with him in court, and I’m looking at a handsome young man, and I’m thinking: ‘Why would you need to go out and track these young women and murder them?’”