HUNTSVILLE — Serial killer Tommy Lynn Sells - a drifter who has been linked to the deaths of more than a dozen people, including a 9-year-old girl - was executed Thursday, despite appeals from attorneys who raised concerns that a secretly made drug used to kill him could deliver a painful death.

Attorneys for Sells had asked the U.S. Supreme Court to block his execution because Texas officials have refused to disclose details about the pentobarbital to be pumped into his body. The high court denied the request.

Sells declined to give a final statement or make eye contact with his victims' families Thursday as he was strapped to a gurney and executed.

Terry Harris, the father of another child victim, broke the silence, commenting as the drugs took effect that the death was "way more gentle than he gave out."

"What a great day," Harris added after Sells was covered with a blanket and the families were escorted out of the viewing area.

Texas Department of Criminal Justice officials pronounced Sells dead at 6:27 p.m., about 13 minutes after he was injected with a fatal dose of the drug.

A Val Verde County jury sent Sells, 49, to death row in 2000 for the December 1999 stabbing death of 13-year-old Kaylene Harris in her family's trailer home near Del Rio. He confessed after a friend who was sleeping over that night survived having her own throat slit and helped identify him to authorities.

He later pleaded guilty in Bexar County to strangling 9-year-old Mary Beatrice Perez, who was abducted in 1999. The missing child, who loved to dance and preferred to be called Mary Bea, was found dead in a creek bed a week later, clad only in a Mickey Mouse T-shirt and a single white sock.

Bexar County District Attorney Susan Reed agreed to drop her bid for a second death sentence, instead settling on life in prison, in exchange for Sells' plea.

As witnesses watched the execution from an enclosed viewing area feet away from his gurney, Sells smiled faintly at two friends there. He closed his eyes and gasped as the drug was administered. A chaplain holding a Bible stood near his feet, clutching his right ankle.

"Whatever went through his veins, he went too quick for my satisfaction," Mary Bea's grandmother, Mary Torres, later said outside the corrections facility, where families of both girls shook hands and embraced.

"I wanted to see him die," added Shawn Harris, Kaylene's brother. "That's honest. I wanted to know that he could no longer hurt anybody."

In the three days leading up to his execution, Sells spent much of the time talking to visitors, prison officials said. He packed his personal property early Thursday morning and was described as reserved. His last breakfast consisted of three pancakes, oatmeal and apple sauce.

The execution came despite last-minute litigation by attorneys for Sells and another death row inmate seeking to have the U.S. Supreme Court intervene because Texas prison officials have refused to disclose details about its newest batch of lethal drugs.

Lawyers for Sells said wanted to know more about how the drug is manufactured in order to evaluate whether it would result in a cruel and unusual punishment.

"It is our belief that how we choose to execute prisoners reflects on us as a society," said a statement released by Sells' lawyers, Maurie Levin and Jonathan Ross, moments after the high court's decision was released. "Without transparency about lethal injections, particularly the source and purity of the drugs to be used, it is impossible to ensure that executions are humane and constitutional."

Texas officials contended that they are not required to disclose details about the drug suppliers or other information because to do so would breach security of the supply chain and could subject suppliers to harassment and threats. They said there is no evidence pointing to the likelihood of severe pain.

The families of both slain children were on a list to witness the execution. Kaylene's witnesses included her father, brother and two grandmothers. Also present were the mother and grandmother of Mary.

Some members of his victims' families said his death was far from cruel compared to how he treated others. Others said they simply didn't care whether Sells suffered.

"We all have suffered so many years," said John Torres, Mary Bea's grandfather. "It's payback time."

Houston Chronicle staff writers Dane Schiller and Mike Ward contributed to this report.