HUNTSVILLE, Texas (AP) — A federal appeals court has halted the scheduled execution of a convicted killer in Texas so his attorneys can pursue appeals he is mentally impaired and ineligible for the death penalty.

Robert James Campbell had been set to die Tuesday evening. He would have been the first U.S. inmate executed since a botched execution in Oklahoma two weeks ago.

His appeals challenged the state's plan to use a drug for which it will not reveal the source, as was the case with drugs used in Oklahoma, and claims of mental impairment.

The 5th U.S. Circuit Court of Appeals halted his punishment about 2 ½ hours before he could have been taken to the Texas death chamber, saying Campbell and his lawyers haven't had a fair opportunity to develop the mental impairment claims.