Just after 9 p.m. (0100 UTC), Virginia executed 49-year-old Alfredo Prieto by lethal injection.

The Supreme Court permitted the execution despite pleas by lawyers that pentobarbital, the execution drug, would insufficiently anesthetize Prieto, causing him "gratuitous and unnecessary pain." They also argued that the prisoner's low IQ of 66 exempted him from the death penalty.

"The lawsuit argues that use of the purported pentobarbital imposes an exceptional and entirely unnecessary risk of a cruel and painful execution," Prieto's lawyers said in a news release before his execution on Thursday.

The lawyers said the pentobarbital came from an "unknown compounding pharmacy" and that the Food and Drug Administration had not assessed it "for quality or authenticity."

The office of Virginia Attorney General Mark Herring had urged the judge to dismiss Prieto's case, arguing that Texas, which has carried out 528 executions since 1982 - more than any other US state - had used the same drugs with success 24 times over the past two years.

The district attorney's office argued that further delaying Prieto's execution and allowing him to "fully indulge his speculations" could prolong the case past the drug's expiration date, and US District Judge Henry Hudson ruled that prison officials had transported and stored the pentobarbital according to the proper standards.

Prieto, a native of El Salvador, was on death row in California for raping and murdering a 15-year-old girl when DNA evidence linked him to the rape and murder of Rachael Raver and the slaying of her boyfriend, Warren Fulton III, in Virginia in 1988.

Authorities have linked Prieto to several other killings, but he was never prosecuted because he had already been sentenced to death.

Prieto's execution came as Oklahoma's attorney general sought to stay three executions to examine the cause of a mix-up with its lethal drugs.

The United States is one of 58 countries to still regularly employ capital punishment and has put 21 people to death so far this year. Before Prieto's execution late Thursday, the most recent had been in the first minutes of Wednesday morning in the state of Georgia.

mkg/jm (Reuters, AFP, AP)