The ex-boyfriend of a missing St. Louis County woman was arrested last week and charged with her death after he admitted to killing her during a physical altercation, police said.

The woman, Marquisha Williams, was reported missing by her family on Dec. 20, the St. Louis County Police Department said in a news release. Her relatives told detectives she was last seen with her ex-boyfriend, Trenton Ivy, in her Jeep Compass.

Two days after her disappearance, Ivy was arrested in Racine, Wisconsin, on charges of resisting an officer, fleeing an officer in a vehicle and using a dangerous weapon. Authorities said he was in Williams’ vehicle and that there was “blood and biological material” in the Jeep as well as her personal belongings.

Williams has not been found.

During an interview with Racine police, Ivy allegedly “admitted to killing Williams in St. Louis,” the news release states.

“The defendant further admitted to having a physical altercation with Williams, which resulted in her death. He then admitted to disposing of her body in an unknown location,” police said.

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Williams was a mother of four children, her aunt told NBC affiliate KSDK of St. Louis.

“Marquisha was very, very sweet, calm and nice,” Sandy Bagley said. “She loved her kids and we all loved being around her.”

Bagley said that on the day Williams went missing, surveillance video showed Williams and Ivy drop off their 2-year-old son at a day care center. The pair had broken up earlier this year and were trying to co-parent, she said.

The family became suspicious when Williams failed to pick up her children.

“We knew something was wrong, she is always on time when she picks up her children,” Bagley told the news station. “We searched in the woods and all along Interstate 55. We just want to find her, lay her to rest and try to start healing. We want Trent to tell us, where is Marquisha?”

Ivy, 31, is being held at the Racine County Jail. He faces a first-degree murder charge in St. Louis County.