No sentencing date was immediately set by a federal magistrate in Tyler. Berkley, who pleaded guilty Wednesday, faces up to five years in federal prison.
Investigators say Berkley last June was in the Hodge Unit when he wrote a letter claiming, when he's released, he planned to travel to Washington in order to kill the president and his family. Berkley was indicted last July.
A spokesman for the Texas Department of Criminal Justice did not immediately return a message Friday from The Associated Press to provide details on Berkley's state imprisonment.