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A former methamphetamine dealer has been executed for killing three people in rural northern Missouri. Fifty-four-year-old John Middleton was put to death Wednesday in the sixth execution in Missouri this year. Only Florida and Texas, with seven each, have performed more.

Middleton was convicted of killing Randy "Happy" Hamilton, Stacey Hodge and Alfred Pinegar in 1995. Authorities say he killed the three out of fear that they would report his drug activity to police. His girlfriend is serving life in prison after pleading guilty to second-degree murder in all three cases.

Middleton's attorneys claimed he was innocent, citing a new witness who came forward in February saying that rival meth dealers, not Middleton, were responsible for the killings. They also claimed he was mentally ill.

Death row inmate John Middleton, 54, is seen in this handout picture taken Feb. 18.HANDOUT / Missouri Department of Corrections via Reuters