I am speaking about Texas Penal Code 25.03 which is. INTERFERENCE WITH CHILD CUSTODY:
"A person commits an offense if he takes or retains a child younger than 18 years when he knows that his taking or retention violates the express terms of a judgment or order of a court disposing of the child's custody. An offense under this section is a state jail felony. "
Reference: http://www.bakers-legal-pages.com/pc/2503.htm
Now to me it seems pretty obvious that this is a criminal offense. I know that in some other states, this offense is expressly a criminal act although it CAN be pursued in civil court too. From a practical standpoint, many police departments tell you they will not get involved with custody cases and want you to hire a private attorney vs. their actually enforcing the law. And this is why custody and visitation orders are violated sometimes with absolute abandon. There is little enforcement. But is this a CRIME if it is specifically outlined in the Texas Penal Code?

