After 24-year-old New Yorker Rebecca Koster goes missing, her family received some very strange text messages from her phone.

Rebecca Koster is out at a bar with her boyfriend in December 2009.

"She was a pretty young woman and she liked to have a good time," said New York Newsday Reporter Andrew Smith.

Then another man hits on the gregarious beauty.

"Exchanging numbers and, putting his number in her phone," said Koster's best friend, Nicole Longo.

And Rebecca Koster mysteriously vanishes that very same night -- until she turns up murdered in the most gruesome of ways.

To die the way she did, as young as she was -- just 24 and so full of life -- made Rebecca's murder all the more tragic.

"Rebecca had a blast anytime she went out," said Khalid, her uncle. "She enjoyed herself to the fullest, which means everyone else did too."

Such is the case when Rebecca, a home health care aide in Medford, on New York's Long Island, steps out to have some fun at some local bars with boyfriend Dan Mayor, her best friend Nicole Longo, and a couple of other friends.

"Yeah, we had a drink and a good time," said Nicole. "She was fine. Everything was fine."

Nicole leaves early, not knowing it would be the last time she would see lifelong friend Rebecca alive.

It's in the early hours of the morning that a mysterious stranger strikes up a conversation with Rebecca as she's getting a drink. The encounter is captured on surveillance video that police have not released, but which Rebecca's family has seen.

"We could see him interacting with her. At one point in the video, he takes her phone and puts his number in it," said Khalid.

"What we know is that Dan and the two friends dropped Rebecca home, and she text messaged her mother," said Andrew Smith.

Her mother, Barbara, is a nurse working the night shift. She had asked Rebecca to let her know she got home safely. And all seems normal when mom Barbara gets home in the morning.

"Her car, her pocketbook is there," said Barbara.

But Rebecca is not, nor is her cellphone. And when she doesn't answer texts or turn up for work, Barbara gets worried and talks to Rebecca's boyfriend, Dan.

"He assured me he dropped Becky off around 3, saw her into the house," said Barbara. "That's the last time he saw her."

The police are notified Rebecca is missing, and family and friends begin their own search for her, guided by a computer device that tracks the location of her phone.

"We made flyers and we searched," said Nicole.

"We were knocking on doors, looking, we couldn't find her," said Barbara.

Then, three days after Rebecca's disappearance, Barbara gets a text message from her daughter's phone.

"That Dan has her. She doesn't know where she is. She needs help," said Barbara. "Right away I panicked, I called her back. 'Dial 911.'"

The search party of friends and family goes straight to Dan's house, arriving at the same time as the police.

"She wasn't there," said Barbara.

Then, just a few hours later, Barbara gets another text from Rebecca's phone.

"It said Dan had her tied up in a basement," said Barbara. "'Don't tell Dan or he'll kill me.'"

And friends and family immediately follow the phone pings, converging on the industrial area of Commack, New York, about 20 miles from their Medford home.

Barbara is distraught. Neither the family nor the police can fathom why Rebecca would text her mother instead of calling or texting 911.

Barbara clings to the hope her daughter is still alive.

Then, tragic news: Rebecca's body is found mutilated and burned beyond recognition, hiding all clues to her killer's identity -- except for one he inadvertently leaves behind.

Just a day after 24-year-old Rebecca Koster mysteriously vanished, a Jane Doe was found murdered in a field in North Stonington, Connecticut, just 85 miles from Rebecca's home in Medford, Long Island.

The body had been stabbed and horribly mutilated, and burned beyond recognition

"It was difficult to identify because all the fingertips were missing, all the toes were missing, her face had been cut off, her nose and ears, anything identifiable was gone," said Newsday reporter Andrew Smith.

Except for the teeth.

"Ultimately that's how they made the identification, through dental records," said Smith.

And six days after her disappearance comes the tragic news that the Jane Doe is Rebecca Koster.

"That's a demon that does something like that," said Barbara, Rebecca's mother.

Friends and family think that demon could be Rebecca's boyfriend of just a few months: Dan Mayor.

Dan is one of the last to see Rebecca before she vanishes, dropping her home that night. And it is Dan who some believed to be holding her captive in a basement, based on texts sent from Rebecca's cellphone to her mother.

But New York Newsday Reporter Andrew Smith says investigators believed he was innocent.

"The police were satisfied that he was telling the truth and that Becky was not there," said Smith.

Andrew Smith says police learned from Rebecca's phone records that she'd received two calls from someone else about an hour after Dan had dropped her home, and just before she disappeared.

"They have his phone number and they figure out who that phone number belongs to, and figure out where that phone has been, and it leads detectives to him," said Smith.

"He" is 33-year-old Evans Ganthier, the mystery man who had been seen exchanging phone numbers with Rebecca in the bar.

"He denies having anything to do with it. At that point they let him go because police believe they don't have enough to hold him," said Smith.

Andrew Smith says police obtained search warrants for Ganthier's home and SUV.

"They find blood in various parts of that car," said Smith.

And although they don't find blood in the house,

"They do find indications that there has been some recent cleaning there on the garage floor," said Smith.

Ganthier reportedly admits he called Rebecca about an hour after she left the bar and claims,

"That he brought her back to his place and she started to get ill," said Smith. "She started to get a cough and foam at the mouth, and then she tripped on some dumbbells in the garage and went down and was unconscious."

According to Smith, Ganthier tells police that discovering Rebecca is dead, he panics, mutilates and dismembers her body to prevent identification, wraps it in plastic bags and a blanket, puts it in the trunk of his SUV, then drives onto a car-carrying ferry near his home in Port Jefferson, and crosses Long Island Sound to Connecticut, where he dumps Rebecca's remains and sets them on fire.

"And he's seen on some surveillance video on the ferry walking around like any other passenger, as if nothing were wrong," said Smith.

Police don't believe Ganthier's claim that Rebecca's death was an accident, and they say that he stabbed her to death.

"Investigators believe Rebecca was murdered in his car," said Smith. "Simply because of the amount of blood in his car and where it was found. There was blood pooled sort of beneath the seat near the center console."

And police reportedly have all the evidence they need to charge Ganthier with Rebecca's murder when forensics experts uncover one tiny vital clue that he failed to erase by mutilating and setting her body on fire.

"They dust the tape that the body was found in and they find a fingerprint on the duct tape, and it comes back to him," said Smith.

Ganthier is charged with second-degree murder, and is ultimately convicted and sentenced to the maximum of 25 years to life.

But that's still not much comfort to Rebecca's grieving family and friends.

"But it doesn't make it right," said Khalid, Rebecca's uncle. "He could've gotten 100 years to life and that wouldn't make it right. Yes, he's behind bars, he can't do this to anyone else, but I hope he suffers every year behind bars."

Rebecca's mom Barbara, who refers to Ganthier as "It," vows she will do all she can to see that he is never released.

"'It' can't hurt anybody right now," said Barbara. "When 'It' comes up for parole, I will be there. We'll all be there."

And Barbara, who still wears a photo of her and Rebecca around her neck, has a warning for other trusting young people like her daughter.

"There are monsters out there and they look like people," said Barbara. "Becky trusted and she paid dearly for it. And if I could reach just one person, male or female, make them think twice about trusting someone they don't know, and save their life, then she'd didn't die in vain."

Ganthier is eligible for parole in 2035. The Koster family promises they will be there to make sure he never walks the streets again as a free man.