Mills was found dead Sunday, one day after his 70th birthday, in the cellblocks at the U.S. Penitentiary in Florence, Colorado, an administrative maximum-security prison called “Supermax,” according to media accounts.

Fremont County Coroner Randy Keller said his death didn’t appear to be suspicious, that Mills apparently died from natural causes, the Pueblo Chieftain reported. However, an autopsy was ordered.

The coroner said if a cause of death isn’t revealed at autopsy, follow-up toxicology tests will attempt to determine if drugs or alcohol were present in Mills’ system at the time of death, the Colorado newspaper reported.

Mills spent most of his life behind bars, directing activities of a white supremacy gang that the FBI says is the most-dangerous group of its kind in the United States.

When he was only 21, Mills, from Windsor, California, was convicted of armed robbery in 1969 and sentenced to a term at San Quentin prison. It was in that California prison where the Aryan Brotherhood gang was formed only a few years earlier – later spreading to other state and federal prisons.

In a 2012 interview with the Intelligence Report, another founding member of the Aryan Brotherhood (AB) described Mills’ instrumental role in the expansion of the prison gang.

His involvement transformed a 50 to 60-member state-prison gang to more than 20,000 members throughout various state and federal prisons, making it the largest and most-dangerous prison-based syndicate.

What’s more, the transformation not only expanded the gang’s influence in various prison systems, but also helped establish channels to direct criminal activities on the streets of various communities.

In short order, the FBI came to describe the Aryan Brotherhood as a violent, white-supremacy gang that is “highly structured” in its leadership roles.

In 2002, as part of an intense effort by the Justice Department to break up the white supremacy gang, prosecutors obtained a multi-count racketeering indictment against 40 of its suspected members. Mills was the lead defendant in the landmark prosecution.

In 2006, Mills and Tyler “The Hulk” Bingham were convicted in U.S. District Court in Santa Ana, California, of murder and racketeering.

During the five-month trial, jurors were told Mills ordered dozens of bloody attacks while he was in custody in maximum-security cellblocks, the Los Angeles Times reported. One of the attacks was an attempted decapitation murder in a federal prison in Atlanta.

Federal prosecutors failed in their attempt to convince the jury that Mills’ actions warranted the death penalty.

Living in the glow of what the Times described as his “near-mythic” reputation and influence, Mills was returned to Supermax where he lived out the final years of his life.

Photo illustration by SPLC