Sherin, reported missing on October 7, was found dead in a culvert about 1 km from her home on October 22 by a cadaver dog during a massive search for the missing toddler.
"We can't go into details, but based on that autopsy report, we were able to determine that we can seek capital murder for this case," Dallas County District Attorney Faith Johnson said at a news conference announcing the indictment.
Wesley, the toddler's adoptive father, was also indicted by a Dallas County grand jury on charges of abandoning a child and tampering with evidence.
Wesley had initially told police that Sherin disappeared overnight after he ordered her to stand outside at about 3 am for not drinking her milk. When he returned, Wesley said she had disappeared and that her location was unknown.
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The Medical Examiner's office ruled last week the toddler died of "homicidal violence."
Prosecutors are not revealing what else they may know about how the little girl was killed.
In a statement today, Mitchell Nolte and Gregg Gibbs, attorneys for Sini Mathews, said, "The indictments handed down today by a Dallas County Grand Jury confirm what we have previously stated. Sini Mathews had nothing to do with the tragic death of Sherin Mathews."
Wesley was arrested and charged with felony injury to a child, which carries a sentence of up to life in prison. He is still in the Dallas County jail on a USD 1 million bond.