U.S. authorities have carried out the 20th execution this year of a condemned prisoner.

Prison officials in Texas executed Gayland Bradford, a 42-year-old man convicted of the 1988 killing of a security guard during a grocery store robbery in which he and a partner fled with only $7. Bradford had spent 22 years on death row.

In carrying out the execution, the Texas prison authorities for the second time used an anesthetic normally reserved for euthanizing animals — pentobarbital — as part of the three-drug formula for the lethal injection because of a U.S. shortage of the drug normally used, thiopental.

The U.S. reinstated the use of the death penalty in 1976, and since then Texas has executed more than four times as many prisoners as any of the other 49 states. Three more executions are scheduled later this month in Texas.