The same jury convicted Barry "The Baron" Mills and Tyler "The Hulk" Bingham. Jurors were then asked, in a separate proceeding, to determine whether Mills, 58, and Bingham, 59 should be sentenced to death or life in prison without possibility of parole.
After 3½ days of deliberations, they told the judge Thursday they were deadlocked, but he ordered them to return to the jury room and try to resolve their differences. A day later, the panelists said they would not be able to reach a unanimous decision.
"We are honestly and conscientiously unable to agree after a full consideration of the evidence," they said in a note sent to Judge David O. Carter.
The judge declared a mistrial in the sentencing phase of the trial, meaning Mills, 58, and Bingham, 59, will serve life in prison without parole for murder, racketeering and conspiracy. Fellow Aryan Brotherhood members Edgar "The Snail" Hevle and Christopher Overton Gibson were also convicted with Mills and Tyler.
Sentencing for Hevle and Gibson was set for Oct. 23. They face 20 years to life in prison.
The case against Mills and Bingham was part of a larger indictment that federal prosecutors hope will dismantle the violent white supremacist organization, which is accused of running powerful gambling operations and drug rings from inside some of the nation's most notorious prisons.
Experts say the full indictment, which lists 32 murders and attempted murders, makes up one of the largest federal capital punishment cases in U.S. history, with more than a dozen people potentially facing execution. More defendants go on trial in Los Angeles later this year.
Jurors told Carter they were split 9-3 in favor of death for Mills and 8-4 in favor of life in prison for Bingham.
The jurors were not made available for comment. Carter said they would be escorted from the courtroom to their cars by authorities, and their identities would remain secret.
The judge scheduled a formal sentencing hearing for Nov. 13.
Mills, Bingham and Hevle also were convicted of a murder count for the killing of Arva Lee Ray, a prisoner slain at the Lompoc, Calif., penitentiary in 1989.
Mills and Bingham were acquitted of a murder count in the death of another Lompoc inmate, William McKinney, a former Aryan Brotherhood member who was killed in 1993. The other two defendants were not charged with that count.