The mother of a slain Burger King cashier burst into tears Thursday when she saw her daughter's alleged killer for the first time in a New York City courtroom, as prosecutors revealed chilling new details about the crime.

"She wept, she couldn't speak. It was emotionally devastating for her," said civil lawyer Sanford Rubenstein who represents shattered mom Kristie Nieves.

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He sat with Nieves in the gallery during Winston Glynn's arraignment in Manhattan Supreme Court for allegedly gunning down her daughter, Kristal Bayron-Nieves, 19, during a robbery at the Harlem eatery.

Winston Glynn Bayron Nieves new york city murder

Winston Glynn has been arrested in connection to the slaying of a Burger King worker (NYPD  |  Fox Nation's "Crime Stories with Nancy Grace")

Prosecutor Dafna Yoran said shortly before the Jan. 9 murder, Glynn, 30, changed into an all black outfit inside a subway station, donned a ski mask, and circled the fast-food restaurant for a half hour before entering through an employee only gate at about 12:30 a.m.

He allegedly pistol-whipped the store’s manager and a customer before demanding that Bayron-Nieves open the register. He stole about $100 from the first register before ordering her to open the second one. When she said it had no cash in it, Glynn allegedly took a step back and blasted her in the chest, striking her lungs and heart, Yoran said. 

Krystal Bayron-Nieves Burger King murder new york city

Krystal Bayron-Nieves, 19, was shot to death during a robbery while working at Burger King Jan. 9. (Courtesy of Kristie Nieves)

He returned to the same subway location, changed back into his regular clothes, discarded a cellphone he'd stolen from the manager and took off, according to the prosecutor. The next day he dumped the clothes he wore during the alleged murder in a dumpster. 

Glynn, who is being held without bail, pleaded not guilty Thursday to first-degree murder, first-degree robbery and other charges before Justice Laura Ward.

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Glynn was a former employee at the Harlem franchise and did not know Bayron-Nieves, an aspiring model who had only been working there for three weeks. The alleged killer, who has a lengthy rap sheet, had been living at a homeless shelter, according to police.

Krystal Baynor-Nieves was gunned down Jan. 9 at Burger King in Harlem

Krystal Bayron-Nieves, 19, was shot to death while working at Burger King during a robbery. (Courtesy of Kristie Nieves)

Manhattan DA Alvin Bragg released a statement Thursday expressing his condolences to the victim's family and saying his office would focus on gun crimes. 

"While we can never bring [Bayron-Nieves] back, or fill the hole left in her loved ones’ hearts, this indictment brings us one step closer to securing justice for her and for her family," Bragg said. "New York City is in the midst of a gun crisis, and tackling gun crime with important prosecutions like these is the top priority for my office."

Bayron-Nieves wasn’t supposed to be on the schedule that Saturday night, her mother previously told Fox News Digital. "She didn’t feel right but her manager told her to come in," Nieves had said. 

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The teen was afraid to work the late shift because of the number of homeless people at the location and the lack of security, her mom had said.

"This is a nightmare that I never thought I’d have to live with, and I never thought it would happen here in America," Nieves told Fox News Digital.

Rubenstein said he's preparing a wrongful death suit against Burger King.

A spokesperson for Burger King said they don't comment on potential litigation. "Our hearts go out to the family, friends and colleagues of the team member involved in the tragic event that occurred at this location," the spokesperson said.