MESA, Ariz. (AP) -- A Mesa woman faces a second-degree murder charge after leading authorities to the shallow grave of a 3-year-old boy in her care, police said.

Heather Nicole Miller, 22, the girlfriend of the boy's father, told investigators she killed Isaiah Vargas Wednesday in her apartment. According to police, Miller said she grabbed his arm and pushed him into his bedroom after he refused to go.

"The defendant said she thought the victim fell down and struck his head on the floor," Detective Dominick Kaufman wrote in court paperwork. "The defendant then grabbed the victim and shook him."

Police said Miller tried to cover up the crime by burying him near a child daycare center, then fabricating a kidnap story.

Miller reported that when she arrived Thursday at her apartment two men jumped out of a car, one assaulted her, and the other took Isaiah.

The FBI, U.S. Marshals Service, and the National Center for Missing and Exploited Children became involved in the investigation.

"Through investigation and follow-up with several people involved in this case, several inconsistencies in Miller's account of her whereabouts, as well as the kidnapping, became apparent," Mesa police spokeswoman Diana Tapia said.

Isaiah was the center of a custody battle between his father Tyrece Young and mother Sylvia Vargas, according to court documents.

Young was awarded custody in May.

In November, Vargas' mother, Maria Robinson, 40, stabbed Miller after they argued about taking Isaiah, according to a police report.

Robinson pleaded guilty to aggravated assault and is to be sentenced April 12.