San Antonio police on Thursday night arrested the boy's step-grandmother, Gloria Proo, on a charge of felony injury to a child.
Police say Josiah Williams on Dec. 27 was found dead at the family's San Antonio home. An autopsy indicated abuse and malnutrition. Investigators say the child had two black eyes and other cuts and bruises.
The boy's father, Charleston Williams, and stepmother Crystal Williams took custody of Josiah after his biological mother went to prison. Police last week charged the couple with injury to a child. Sgt. Jim Cole says Proo is the mother of Crystal Williams.
Online jail records didn't immediately list information on Proo.
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