Frazier Glenn Cross Jr.

Updated at 11:51 a.m. Tuesday

A man who testified that he wanted to kill Jews and was sentenced to death after he shot and killed three people at Jewish sites in suburban Kansas City in 2014 has died in prison, the Kansas Department of Corrections said Tuesday.

Frazier Glenn Miller Jr., 80, died Monday at the El Dorado Correctional Facility, where he was serving a sentence for capital murder, attempted murder, assault and firearms convictions.

An autopsy will be performed to determine a cause of death, but preliminary indications were that Miller died of natural causes, Carol Pitts, a spokeswoman for the corrections department, said in a news release. She declined to comment further on Miller’s death or medical condition.

In March, Miller’s attorneys argued before the Kansas Supreme Court that his death sentence should be overturned, in part because they said he should not have been allowed to represent himself at his trial.

Miller drove from his home in Aurora, Mo., determined to kill Jews. On April 13, 2014, he ambushed William Corporon, 69; and his 14-year-old grandson, Reat Underwood, at the Jewish Community Center in Overland Park, Kan. He then drove to the nearby Village Shalom care center and killed Terri LaManno. None of the victims were Jewish.

Miller, also known as Frazier Glenn Cross, testified during his trial that he wanted to kill Jewish people before he died. He said he didn’t expect to live long because he had chronic emphysema.

During his trial and at his sentencing, Miller frequently interrupted the proceedings to give rambling statements about his belief that Jewish people were running the government, media and the Federal Reserve.

During his closing arguments at trial, Miller said he had been “floating on a cloud” since the killings. When he was convicted and when he was sentenced to death, Miller raised his arm in the Nazi salute.

Miller was a Vietnam War veteran who founded the Carolina Knights of the Ku Klux Klan in his native North Carolina and later the White Patriot Party. He also ran on a white power platform during campaigns for the U.S. House in 2006 and the U.S. Senate in 2010 in Missouri.

In arguments before the Kansas Supreme Court in March appealing his death sentence, Miller’s attorneys argued that he was incapable of understanding the legal intricacies of a complicated death penalty case and should not have been allowed to represent himself, even though he insisted on being his own attorney.

Attorney Reid Nelson said Miller’s standby attorneys should have been allowed to intervene during the penalty phase.