PHOENIX, March 29 - A man who shot dead two convenience store workers in a robbery in 1989 was executed by lethal injection on Tuesday, after a legal challenge raising concerns over the drugs used in the execution failed.

A jury convicted Eric John King, 47, on two counts of first degree murder for shooting convenience store clerk Ron Barman and the store's security guard Richard Butts during a robbery in December 1989 that netted $72.

King was executed at a state prison in Florence, southeast of Phoenix, Arizona Attorney General Tom Horne said in a statement. Time of death was given as 10:22 a.m. local.

"This morning the state of Arizona fulfilled its most somber obligation to protect public safety by executing convicted murder Eric King by lethal injection," Horne said, adding that he was a witness to the execution.

King was the first prisoner to be put to death by Arizona this year.

Lawyers for the death row inmate had filed a motion seeking a stay of execution pending an investigation into the way Arizona acquired two of the three drugs the state uses in lethal injection.

They argued that the Arizona Department of Corrections violated state law when it imported its supply of pancuronium bromide and sodium thiopental, which it had described in import documents as being for animal use.

The appeal was denied by the state Supreme Court.

There are 130 people on death row in Arizona, which has executed 25 people since the death penalty was restored in 1992.

The last execution prior to King's was in October 2010.

(Reporting by Tim Gaynor and David Schwartz; Editing by Jerry Norton)