The Mexican citizen executed Wednesday night for killing his 16-year-old cousin wrote in a handwritten statement that he will be back for justice, "You can count on that!"

Ruben Ramirez Cardenas, 47, was given a lethal injection after several federal court appeals failed to halt his punishment for the February 1997 killing of Mayra Laguna, 16. She was abducted from her family's apartment and beaten to death.

Asked by the warden to make a final statement, he replied, "No, sir."

As the lethal dose of pentobarbital began, he took a couple of breaths and then began snoring. After less than a minute, all movement stopped.

Twenty-one minutes later, at 10:26 p.m. CST, he was pronounced dead, making him the seventh convicted killer put to death this year in Texas, which carries out capital punishment more than any other state.

In a handwritten statement released afterward, Cardenas thanked his family, attorneys and the Mexican consulate for their help.

His punishment was delayed for about four hours as last-ditch appeals for the former security guard focused primarily on efforts to have trial evidence undergo new DNA testing. In a filing to the U.S. Supreme Court hours before his execution, lawyers argued Texas was violating Cardenas' due process rights and a state statute that covers forensic testing. They asked the justices to halt the execution for a court review.

They also asked the justices for more time to appeal a lower court's rejection of a federal civil rights lawsuit in which they claimed his due process and civil rights were being violated because Texas officials wouldn't release evidence so it can undergo new DNA testing.

Attorneys for the state said the lawsuit was improper and that state courts already refused the DNA request because Cardenas could not show that more advanced tests would exonerate him. DNA results in evidence at Cardenas' trial were not false, state attorneys said.

The high court, without comment, rejected both appeals.

Earlier this week, the Texas Court of Criminal Appeals, the state's highest criminal court, rejected a similar appeal seeking DNA tests. Cardenas' attorneys argued the new testing would be better than the now-obsolete testing that left "persistent doubts about his guilt and the integrity of his conviction."

Laguna was snatched from a bedroom she shared with a younger sister at her family's public housing apartment in McAllen in South Texas. In a confession to police, Cardenas said he and a friend drove around with the high school student in his mother's car. He said he had sex with the teen and then punched her as she fought him after he unbound her arms to let her go.

"I didn't plan on doing this, but I was high on cocaine," he told authorities.

He said after he hit the teen in the neck, she began coughing up blood and having difficulty breathing. After trying unsuccessfully to revive her, he said he tied her up "and rolled her down a canal bank."

Her body was found in a canal near a lake in the Rio Grande Valley in far South Texas. It’s what happened next that has Mexican officials and human rights groups arguing he shouldn’t be put to death.

Cardenas' defense team, funded by the Mexican government, claims the United States violated an international treaty by denying Cardenas the opportunity to speak to his country’s consulate after his arrest.

“It’s a significant treaty violation,” Gregory Kuykendall, an Arizona attorney authorized to speak on behalf of Mexico, told the Houston Chronicle.

If Texas officials execute Cardenas, they would be in apparent violation of the Vienna Convention on Consular Relations.

In 2004, the U.N.’s international court in the Hague ruled that the U.S. must review cases similar to Cardenas’. Then-President George W. Bush told states to comply but the Supreme Court overruled him and said it was up to Congress – not the president – to order states to obey.

Since then, Congress has dragged its feet.

“The result: an illegal loophole states are exploiting to execute foreign nationals in violation of international law,” the ACLU says.

Laguna's sister, Roxana Jones, said she had waited 21 years for justice to be served.

"Words can't begin describe the relief it feels to know that there is true peace after so much pain and sorrow," she said in a statement.

Cardenas' attorney, Maurie Levin, contended eyewitness testimony against Cardenas was shaky, that little physical evidence tied him to the killing and that his confession was obtained after 22 hours of isolation and intense police questioning.

She also said that authorities acted improperly when not telling the Mexican-born Cardenas that he could get legal help from the Mexican consulate.

Cardenas grew up in the Texas Rio Grande Valley.

The friend who was with Cardenas during the abduction, Jose Antonio Lopez Castillo, now 45, was convicted of aggravated kidnapping and is serving a 25-year prison term.

Fox News' Barnini Chakraborty and The Associated Press contributed to this report