Missouri executed an inmate Wednesday involved in two murders during a 1998 robbery. He is the eighth convict to be put to death in the state this year and 28th in the United States.

Earl Ringo Jr. was injected with a lethal dose at 12:22 a.m. and was pronounced dead at 12:31 a.m., according to the Department of Corrections. His attorneys sought postponement of the execution until a hearing over Missouri's use of midazolam, a controversial sedative, which the state has been using since last November.

According to attorney Richard Sindel, use of midazolam violates the state's protocol that provides only pentobarbital drug for execution, reports the Associated Press.

However, the U.S. Court of Appeals for the Eighth Circuit denied the appeal Tuesday. The U.S. Supreme Court late Tuesday also denied requests to stay the execution.

Ringo's attorney, Kay Parish, had sought a stay on his execution, citing a St. Louis Public Radio report that state officials had been less than truthful about the drugs used in recent executions.

St. Louis Public Radio reported last week that Missouri administered midazolam to all nine inmates put to death since November. In defense, David Owen, the corrections department spokesman said, midazolam "relives the inmate's level of anxiety and is not part of the actual execution process."

Talking about the execution, Missouri's Attorney General Chris Koster said that Ringo Jr. was a murderer of two innocent Missourians. "For 16 years he avoided payment for this crime. Tonight he has paid the penalty," Koster said in a statement, according to Reuters.

Next execution in the United States is set in Texas for Wednesday night. Murder convict Willie Trottie is scheduled to die for killing his common-law wife and her brother in 1993. If executed, he will be the eighth Texas inmate to be put to death this year.