A killer has been executed by lethal injection for the 1980 killings of two people despite claiming he was innocent.
Murray Hooper, 76, was killed at the state prison in Florence, Arizona at around 10.34am local time, for the killings of William "Pat" Redmond and his mother-in-law, Helen Phelps.
According to Arizona Department of Corrections Deputy Director Frank Strada, Arizona executed Hooper 34 minutes late due to difficulties inserting IVs into him.
It is the third execution since Arizona started carrying out the death penalty after nearly an eight-year hiatus.
The killings were carried out at the request of a man who wanted to take over Mr Redmond's printing business, according to authorities.
Hooper's lawyers attempted to delay the execution and demanded fingerprint and DNA testing on evidence from the killings - which the court refused.
The killer claimed he was innocent and said no physical evidence linked him to the killings and that testing could identify the people responsible.
His lawyers argue Hooper was convicted before computerised fingerprint systems and DNA testing were available in criminal cases.
The team even asked the US Supreme Court to review his claim that authorities had until recently withheld that Redmond's wife, Marilyn, who survived being shot in the head during the attack, had failed to identify him in a photo lineup.
However, authorities say that claim is based on a mistake a prosecutor made in a letter to the state's clemency board and now insist that no such lineup was shown to Marilyn Redmond.
She later identified Hooper in an in-person lineup and testified against him at his trial.
Authorities say Hooper and two other men forced their way into the Redmond home on Dec. 31, 1980. The three victims were bound, gagged, robbed and shot in the head.
Two other men, William Bracy and Edward McCall, were convicted in the killings but died before their death sentences could be carried out.
Authorities say Robert Cruz, who was alleged to have had ties to organised crime, hired Hooper, Bracy and McCall to kill Pat Redmond, who co-owned a printing business.
They said Cruz wanted to take over the business and was unhappy that Redmond had rejected his offers to enter several printing contracts with Las Vegas hotels, according to court records.
Cruz was acquitted of murder charges in both deaths in 1995.
Hooper's lawyers say Marilyn Redmond's description of the assailants changed several times before she identified their client, who said he was not in Arizona at the time.
They also raised questions about the benefits received by witnesses who testified against Hooper, including favourable treatment in other criminal cases.
Arizona did not carry out the death penalty for nearly eight years after criticism that a 2014 execution was botched and because it encountered difficulty obtaining lethal injection drugs.
No other executions are currently scheduled in the state.
Arizona has 111 people on death row, 22 of whom have exhausted their appeals, according to the state attorney general's office.
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