Desiree Robinson had recently moved into her grandfather's home, hoping that the change of pace and space on Chicago's Far South Side would slow down her lifestyle.

Her mother, Yvonne Robinson-Ambrose, heard from her daughter sporadically, but couldn't keep track of her. "She wouldn't let us know where she was," she told local media.

Desiree, 16, had begun skipping school at Magic Johnson Academy. Her grandfather, Dennis Treadwell, started hearing things, unbelievable things.

"You don't want to believe these things. A granddaughter that's so vibrant and seeming to want to live life, just kind of misled herself... and got into a bad situation," Treadwell told WGN.

That bad situation turned out to be deadly, police said.

Robinson suffered "multiple injuries from an assault" on Dec. 31, 2016 at a suburban Markham, Illinois home, according to police. But it was more than an assault. It was a murder, Cook County law enforcement officials said.

Her friends discovered her body in a pool of blood inside a garage in the 16200 block of South Hamlin Avenue, authorities said.

Police found a bloody knife in the garage of Antonio Rosales, 32, and charged him with murder. He admitted to the slaying and told authorities that he met the teen online from an ad on Backpage.com, according to the Cook County State's Attorney's Office. 

Authorities said Rosales negotiated sex with Desiree at his home. She came over with two female friends. The friends told police that they saw Robinson and Desiree have sex in a truck.

Soon, Desiree and her friends left, but Rosales called again, asking for Desiree to come back. He wanted to have sex again, prosecutors said.

This time the two friends stayed outside in a car, while Desiree and Rosales went into the garage. According to police, Rosales told them that a fight began when he didn't pay for sex. The altercation escalated, when he punched and strangled her as she tried to call out for her two friends.

The two friends went to sleep through the night, wondering where was Desiree. About 9 a.m., Rosales emerged and knocked on the girls' window, telling them that Desiree would be "out shortly," according to the ChicagoTribune. Soon he left in his vehicle, police said.

Worried, the two friends entered the garage and found Desiree's lifeless body. They called police.

Rosales is a convict. He served two years in an Indiana prison for assault and battery, WGN reports.

"I'm just numb," the girl's mother told KTLA.