A death row inmate who was challenging Alabama's method of lethal injection died Tuesday, apparently of complications from cancer, officials said.

Prison system spokesman Brian Corbett said Daniel Siebert, 53, was pronounced dead at 1:35 p.m. at Holman prison near Atmore, where he had been awaiting execution for more than 21 years for strangling four people.

Siebert, a self-described serial killer, was also known for grim drawings he made that were offered for sale on "murderabilia" Web sites specializing in artwork, letters and essays by convicted killers.

Corbett said the exact cause of Siebert's death would be determined later but it appeared to be related to his pancreatic cancer.

"He certainly hoped to die from the cancer before he was executed," said Esther Brown, executive secretary of Project Hope to Abolish the Death Penalty.

Siebert's death comes less than a week after the U.S. Supreme Court approved the most widely used method of lethal injection, prompting states to move forward with executions after a nearly seven-month halt.

He was one of several death row inmates challenging Alabama's method, but his suit was unique in that he claimed his cancer medication would counteract with the lethal injection drugs and inflict unnecessary pain.

Siebert was condemned for the Feb. 19, 1986, strangulation of his 24-year-old girlfriend, Sherri Weathers, and her two sons, 5-year-old Chad and 4-year-old Joey. He was also convicted separately and sentenced to death for killing a neighbor at her Talladega apartment complex, Linda Jarman, the same night.

Siebert was sentenced to life in prison for killing Linda Faye Odum, also of Talladega. He confessed to a number of other killings from California to New Jersey, but the exact number of his victims is not known.

Siebert came within a day of lethal injection in October before a federal appeals court stayed his execution pending the Supreme Court ruling on a Kentucky case.

Alabama Attorney General Troy King said Siebert's death should "put an end to the years of legal shenanigans that have gone on."

"It's a shame that he got what he wanted, but the people who he brutally executed had no say in the matter at all and that's the injustice of this," King said.

Copyright 2008 Associated Press