Humberto Leal Garcia Jr.'s likely last hope is with Texas Gov. Rick Perry, who is still considering whether to grant a 30-day reprieve. Leal's lethal injection is set for 7 p.m. ET Thursday at the Texas State Penitentiary's death chamber in Huntsville.
Texas authorities said they gave Leal his last meal Thursday afternoon as hours dwindled before his scheduled execution.
Perry was expected to decide Thursday whether to grant clemency or any delay in the execution of Leal, whose case has become the center of an international legal and diplomatic dispute.
The Obama administration and international leaders have asked for a reprieve on behalf of the inmate.
The victim's mother, Rachel Terry, has urged the execution go forward as scheduled.
What makes Leal's conviction unusual is that he was not informed about his right to contact the Mexican consulate upon his arrest -- a right guaranteed under a binding international treaty. Leal's appellate lawyers argue such access could at the very least have kept Leal off death row.
"A technicality doesn't give anyone a right to come to this country and rape, torture and murder anyone, in this case my daughter," she told CNN affiliate KSAT in San Antonio. Terry described her daughter as "a beautiful, bright, vibrant young woman, full of hope and aspirations."
"It's been difficult for myself and her family members," she added. "She certainly was taken away from us at a very young age. We just want closure."
Leal's lawyers argue the consulate access violation was more than a technicality. Sandra Babcock, lead appellate attorney, told CNN that Mexican officials would have ensured Leal had the most competent trial defense possible, if they had been able to speak with him right after his felony arrest.
"I think in most of these cases it was not a deliberate thing. Local police lack training" on the Vienna Convention, Babcock said.
Leal's backers say he has learning disabilities and brain damage, and suffered from sexual abuse at the hands of his parish priest, and that consul officials would have assisted in such a defense. They say those factors should have been considered at the sentencing phase of the trial.
Leal claimed he did not learn of his consular access right until two years after his capital conviction. He said he learned of the right not from any official, but from a fellow prisoner.
Eventually, between 2010 and 2011, Leal was visited by a representative of the Mexican government more than 10 times, said Judy Garces, press relations spokeswoman with the Mexican Consulate in San Antonio.
The state argued that Leal -- who has lived in the United States since age 2 -- never revealed his Mexican citizenship at the time of arrest, and his defense team never raised the consular access issue at or before trial.
Prosecutors also say the evidence against him was indisputable. The victim was tortured, and a bite mark on her body was matched to Leal. A bloody shirt belonging to Sauceda was discovered at the suspect's home.
The two had attended a party separately earlier that evening. The girl's nude body was found on a dirt road.
Leal is one of 40 Mexican citizens in a similar situation awaiting lethal injection in U.S. prisons.
The issue turns on what role each branch of government plays in giving force to international treaty obligations.
Three years ago, the Supreme Court justices concluded Texas could execute another Mexican man sentenced to death for murder. Jose Medellin was given a lethal injection by the state a few months later.
The question then and now is whether the state has to give in to a demand by the president that the prisoner be allowed new hearings and sentencing. In the 2008 case, President George W. Bush made that demand reluctantly after an international court concluded Medellin was improperly denied access to his consulate before his original prosecution, a violation of the treaty signed by the United States decades ago.
The Mexican government filed a supporting appeal with the high court in Washington this past week, asking the justices to block Leal's execution, citing Bush's previous executive action.
On Friday, the Obama administration asked Texas to delay the execution.
"This case implicates United States foreign policy interests of the highest order," including protecting U.S. citizens abroad and promoting good relations with other countries, new Solicitor General Donald Verrilli Jr. said.
It is rare for the federal government to intervene in cases dealing with state executions.
Congress has also stepped in. Sen. Patrick Leahy, D-Vermont, chairman of the Judiciary Committee, introduced a bill in June to formally grant federal courts the power to review these kinds of appeals.
Perry's office countered that a federal appeals court has already given Leal the judicial scrutiny the Obama administration and the United Nations seek, ultimately rejecting his claims.
"If you commit the most heinous of crimes in Texas, you can expect to face the ultimate penalty under our laws," Katherine Cesinger, spokeswoman for the governor, told CNN.
"Congress has had the opportunity to consider and pass legislation for the federal courts' review of such cases since 2008, and has not done so each time a bill was filed."
Ted Cruz, the state's former solicitor general who argued the 2008 Supreme Court case for Texas, said Leal has waited too long to raise these issues.
"The question is not should a foreign national have the right to contact their consulate," Cruz told CNN. "The question is, years later, after they have been tried, after they have been convicted, after it has been clear like Humberto Leal that they are a vicious child rapist and murderer, should you come in and set aside that conviction. You can't come back years later and try and set aside your trial with some additional claim you wish you had raised."
Cruz is now running for the U.S. Senate as a Republican candidate.
Witnesses to Leal's execution were to include three of his friends, his sister and his attorney, Clark said. No family members of the victim were scheduled to attend, he said. Two former prosecutors who tried the case were also scheduled to be witnesses.
The Supreme Court appeal is Leal v. Texas (11-5001)