When District authorities determined 3-year-old Xavier Lyles had been beaten to death, Xavier's mother told police that she never hit her son and that her boyfriend struck the fatal blows.

Francis Lyles's family members were outraged and wanted to help secure the arrest of Lyles's boyfriend. One of Lyles's cousins told police she had recorded numerous conversations on her cellphone with a recording app and gave prosecutors permission to access them.

The recordings, however, did not implicate Lyles's boyfriend, prosecutors say. Instead, prosecutors contend that Lyles can be heard describing harming the child after he knocked over his newborn brother's bouncy seat while the infant was in it.

"I f---ed Xay up" Lyles says in the recording, according to documents filed in D.C. Superior Court.

That recording and other evidence are expected to be laid out before a jury beginning Monday in Judge Zoe Bush's courtroom in one of the District's most recent cases of a mother charged with killing her son.

Lyles, 28, faces multiple counts of first-degree murder and cruelty to children in the 2014 fatal beating. At the time, Lyles, her three children and her boyfriend lived in a Southeast Washington apartment.

Lyles has continued to assert that she did not hurt Xavier and that he was injured while in the care of her boyfriend. Calls and emails to Lyles's court-appointed attorney Elliott J. Queen were not returned.

But in a court filing, Queen contends that the telephone conversation with the cousin was from June 12, 2014, more than a week before Xavier's death. He added that his client's language did not refer to Lyles giving her son a severe beating and said the discussion had no connection to the boy's injuries.

"The vulgar term is often used in many settings that have nothing to do with violence or the severity of a beating," Queen wrote. "Again, it is not the best choice of words, but, as here will be taken out of context."

[Death of 3-year-old boy ruled homicide]

In a court filing, Assistant U.S. Attorney Cynthia Wright wrote that on June 23, a day before Xavier was killed, neighbors saw the boy playing with his two cousins and sister. At around 5 p.m., prosecutors say Xavier went into the apartment to take a nap. A few hours later, neighbors told prosecutors they heard Xavier inside playing.

Then at around 8:30 p.m., prosecutors say Lyles became angry with Xavier when he ran into the infant's bouncy seat. Wright argues Lyles used so much force that Xavier's liver was lacerated in three places and his kidney was damaged. Neighbors told prosecutors they only heard the voices of Lyles and her son, and never an adult male's voice, at the time, according to court documents.

The next day, on June 24, Lyles went into her son's room where she discovered he was not moving or breathing. Prosecutors say instead of calling 911 for help, Lyles first called her cousin. Xavier was pronounced dead at a nearby hospital.

During the early part of the case, prosecutors had offered Lyles a plea deal but she rejected the offer. She has been jailed since her arrest.