A Texas health agency has begun investigating more than three dozen healthcare providers who prescribed large quantities of powerful psychiatric drugs — some to children — after a U.S. senator raised questions about the medications.

The Texas Health and Human Services Commission has referred three providers to the attorney general for criminal prosecution, state Health Commissioner Thomas Suehs wrote to Sen. Charles Grassley last month. Some have been excluded from the Texas Medicaid program, including one convicted in a criminal case and another accused of inappropriate billing and coding of hours related to patient services.