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Federal authorities have charged a Pine Ridge woman in relation to the gruesome July death of her 2-year-old grandson, who authorities say was beaten to death by her mother over potty-training issues.

The toddler's mother, meanwhile, faces a new charge of assault, on top of the charges of murder and child abuse she already faces.

The boy's mother, Katrina P. Shangreaux, 28, was charged in an updated indictment last week with assault resulting in serious bodily injury of a minor. A grand jury indicted her in August of first-degree murder, felony child abuse, as well as felony child abuse and neglect.

After further investigation, authorities now say Shangreaux’s mother was also complicit in the crime. Sonya E. Dubray, 47, the boy's grandmother, was charged in the same indictment with tampering with evidence, accessory to first-degree murder, making a false statement and misprision of a felony.

The South Dakota U.S. Attorney’s Office charges that Shangreaux killed the toddler with blows to the head and abdomen on July 28 in Porcupine. It also says she seriously injured the child by causing intracranial bleeding and brain damage after hitting him with her fists and feet.