PIERRE, S.D. — An inmate who killed a South Dakota prison guard during a failed escape attempt should face the death penalty, the state Supreme Court ruled Thursday.

Eric Robert, 50, pleaded guilty to killing Ron Johnson in April last year and asked to be executed for the crime.

Circuit Judge Bradley Zell of Sioux Falls sentenced Robert to death and set his execution for May, but the Supreme Court postponed the execution so it could conduct a mandatory review of the sentence.

The high court unanimously upheld the death sentence Thursday. The justices ruled that the penalty is proportionate to the crime and justified by aggravating circumstances – that Robert killed a prison guard and the murder occurred during an escape attempt.

"This was not merely an escape attempt on the spur of the moment where events spiraled out of control. Here the record reflects that Robert had been planning his escape attempt, which included the murder of a corrections officer, for well over a month. His planning stage included obtaining the lead pipe eventually used to kill Johnson," Chief Justice David Gilbertson wrote.

The circuit judge was not influenced by passion or prejudice in sentencing Robert to death, but instead considered that Robert is dangerous because he has threatened to kill again, has a violent history, is unlikely to be rehabilitated and committed a severe crime, the Supreme Court said. The justices said their review of the sentence was particularly important because Robert has demanded to be executed.

"Robert's persistent efforts to hasten his own death necessitate intense scrutiny to guarantee his desire to die was not a consideration in the sentencing determination. We do not participate in a program of state-assisted suicide," Gilbertson wrote.

The high court said the circuit judge can issue another execution order.

South Dakota Attorney General Marty Jackley, who prosecuted the case, said he will ask the circuit judge to set an execution date. Robert's execution by lethal injection is unlikely to occur for at least three or four months, he said.

Robert was already serving the equivalent of a life sentence – 80 years for a kidnapping conviction – so a death sentence is justified because another life term would not have increased his punishment, Jackley said.

"The death sentence is reserved for only the most brutal crimes that are committed. The horrific actions of Robert leave no other alternative and justify the sentence of death," Jackley said.

Defense lawyer Mark Kadi, of Sioux Falls, said Robert seemed relieved that the Supreme Court will allow his execution. He wants it to be carried out soon and will not seek any last-minute delays, Kadi said.

"Just as he has not taken steps to pursue appeals in his case, he will not seek or accept any executive clemency," Kadi said.

A second inmate involved in the escape attempt, 49-year-old Rodney Berget, also pleaded guilty and was sentenced to death, but has appealed his conviction and sentence. A third inmate, Michael Nordman, 47, was given a life sentence for providing the plastic wrap and pipe used in the slaying.

Johnson was working alone on April 12, 2011, his 63rd birthday, in a part of the prison where inmates work on upholstery, signs, custom furniture and other projects. Prosecutors said the two inmates killed Johnson by hitting him with a pipe and wrapping his head in plastic wrap. Robert put on Johnson's uniform and tried to move a large box toward the prison gate with Berget inside. The inmates were apprehended before leaving the prison.

The prison made more than a dozen procedural changes after Johnson's killing to improve security and safety.

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