Mexican officials are incensed because Texas has opposed legal efforts and spurned diplomatic pressure to spare a prisoner who was in the United States without legal permission when he was condemned for fatally shooting a Houston police officer two decades ago.

Edgar Arias Tamayo, 46, is set to die by lethal injection at 6 p.m. on Wednesday for the January 1994 slaying of Guy Gaddis, 24.

Gaddis, who'd been on the force for two years, was driving Tamayo and another man from a robbery scene when evidence showed he was shot three times in the head and neck with a pistol Tamayo had concealed in his pants. The car crashed, and Tamayo fled on foot but was captured a few blocks away, still in handcuffs, carrying the robbery victim's watch and wearing the victim's necklace.

Tamayo's attorneys considered last-day attempts to keep him from the death chamber after a federal district judge on Tuesday rejected their lawsuit challenging what they argued was Texas' unfair and secretive clemency process. 

They and the Mexican government also contended Tamayo's case was tainted because he wasn't advised under an international agreement that he could get legal help from his home nation following his arrest.

Secretary of State John Kerry previously asked Texas Attorney General Greg Abbott to delay Tamayo's punishment, saying it "could impact the way American citizens are treated in other countries." The State Department repeated that stance Tuesday.

But Abbott's office and the Harris County district attorney opposed postponing what would be the first execution this year in the nation's most active capital punishment state, where 16 people were put to death in 2013.

The Mexican government said in a statement this week it "strongly opposed" the execution and reminded that failure to review Tamayo's case and reconsider his sentence would be "a clear violation by the United States of its international obligations."

Mexican officials and Tamayo's attorneys contend he was protected under a provision of the 1963 Vienna Convention on Consular Relations. Legal assistance guaranteed under that treaty could have uncovered evidence to contest the capital murder charge or provide evidence to keep Tamayo off death row, they said.

"We are continuing to pursue our options for appeal, and vindication of Mr. Tamayo's right to review of the consular rights violation in his case," said Maurie Levin, one of Tamayo's lawyers.

Tamayo's appeal to a federal court in Austin sought an injunction against Gov. Rick Perry and the Texas Board of Pardons and Paroles, which he appoints. The board can recommend Perry grant clemency, but it's an action they rarely take.

Levin and lawyer Sandra Babcock argued the state's clemency procedures were "an affront to what clemency is supposed to be, a 'failsafe' in our judicial system."

"We are deeply disappointed by the court's ruling," Levin and Babcock said in a statement last night, "but we will continue to fight for Edgar's right to review of his case."

Abbott's office countered the procedures met U.S. Supreme Court guidance.

"It doesn't matter where you're from," Perry spokeswoman Lucy Nashed said. "If you commit a despicable crime like this in Texas, you are subject to our state laws, including a fair trial by jury and the ultimate penalty."

Tamayo was in the U.S. illegally and had a criminal record in California, where he had served time for robbery and was paroled, according to prison records.

"Not one person is claiming the suspect didn't kill Guy Gaddis," Ray Hunt, president of the Houston Police Officers' Union, said. "He had the same rights as you and I would have.

"This has been looked at, heard, examined and it's time for the verdict of the jury to be carried out."

Legal challenges about consular notification and Mexican nationals on Texas death row aren't new. At least two other inmates in similar circumstances were executed in Texas in recent years.

Tamayo was among more than four dozen Mexican nationals awaiting execution in the U.S. when the International Court of Justice in The Hague, Netherlands, ruled in 2004 they hadn't been advised properly of their consular rights. The Supreme Court subsequently said hearings urged by the international court in those inmates' cases could be mandated only if Congress implemented legislation to do so.

"Unfortunately, this legislation has not been adopted," the Mexican foreign ministry acknowledged.

Based on reporting by the Associated Press.

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