Three US justice officials who tackled white supremacist prison gangs have been killed. Originally formed to fight other gangs, these groups are now accused of a range of criminal activities on the outside, from drug smuggling and kidnapping to murder. How did neo-Nazi prisoners set up huge criminal networks?

With skinhead haircuts and swastika tattoos, their leaders are buried deep within the brutal confines of America's penitentiaries.

But three murders in less than three months have shone a spotlight on far-right prison gangs, whose empire of drug-dealing, racketeering and murder extends well beyond the walls and barbed wire around them.

The bodies of Kaufman County, Texas, district attorney Mike McLelland, 63, and his wife Cynthia, 65, were found on Saturday.

McLelland's deputy, Mark Hasse, was killed in January, on the same day it was announced that their office was pursuing a racketeering case against the Aryan Brotherhood of Texas (ABT), a white supremacist group formed in Texan jails.

Police are investigating whether their deaths were linked with the killing of Tom Clements, Colorado's head of prisons.

The chief suspect in that case, ex-convict Evan Ebel, is said to have belonged to the 211 Crew, another violent racist prison gang. Official documents state his body was covered with Nazi-themed tattoos. Ebel died in a shoot-out two days after Clements.

Timeline 9 November 2012: Some 34 alleged Aryan Brotherhood of Texas members indicted by the FBI over three murders, several attempted murders, assault, kidnapping, and conspiracy to distribute methamphetamine and cocaine

Some 34 alleged Aryan Brotherhood of Texas members indicted by the FBI over three murders, several attempted murders, assault, kidnapping, and conspiracy to distribute methamphetamine and cocaine December 2012: Texas Department of Public Safety warns it has information to suggest the ABT is "actively planning retaliation against law enforcement officials"

Texas Department of Public Safety warns it has information to suggest the ABT is "actively planning retaliation against law enforcement officials" 31 January 2013: Kaufman County, Texas, Assistant District Attorney Mark Hasse shot dead on the same day the justice department announces his office is pursuing a racketeering case against the ABT

Kaufman County, Texas, Assistant District Attorney Mark Hasse shot dead on the same day the justice department announces his office is pursuing a racketeering case against the ABT 19 March: Tom Clements, head of Colorado's department of corrections, is killed

Tom Clements, head of Colorado's department of corrections, is killed 21 March: Lead suspect in Clements' killing, Evan Ebel (above), dies in a shootout with police. Ex-convict Ebel had Nazi tattoos and was linked to a far-right prison gang

Lead suspect in Clements' killing, Evan Ebel (above), dies in a shootout with police. Ex-convict Ebel had Nazi tattoos and was linked to a far-right prison gang 30 March: Hasse's boss Mike McLelland, 63, and his wife Cynthia, 65, found shot dead in their home

While the killings remain unsolved, they have focused attention on the increasingly dangerous white supremacist networks formed in prison.

The Anti-Defamation League (ADL), which monitors hate in the US, describes the ABT as "the most violent extremist group in the United States". It says the gang, thought to have around 2,000 members, has committed "at least" 29 murders in the US between 2000-12.

Its primary objective has moved beyond conducting turf wars inside jails or propagating racist ideology, however, into running a ruthless Mafia-style organised crime network.

An FBI indictment in November 2012 charged 34 ABT members with three murders, several attempted murders, assault, kidnapping and conspiracy to distribute methamphetamine and cocaine. According to court papers, the ABT has a tightly organisational structure composed of five regions, each run by a "general."

"If you look at domestic extremist groups in the US, they are responsible for more homicides than anyone else, although most are crime-related, to do with insubordination or revenge or against those who owe them money," says Brian Levin, director of California State University's Center for the Study of Hate and Extremism.

Of the confirmed ABT murders from 2000-12, the ADL estimates that 41% were "internal killings".

Three ABT members have pleaded guilty to the murder of Mark Davis Byrd Jr, who was beaten, tortured stabbed and shot in San Antonio in 2008.

Byrd, who had reportedly embezzled drugs he was supposed to deliver, had two of his fingers cut off - a calling card of the ABT, whose modus operandi dictates that a digit should be removed from its victims as a trophy.

The US's original neo-Nazi prison gang, known simply as the Aryan Brotherhood, emerged in California's San Quentin State Prison during the late 1960s.

Desegregation of American jails meant inmates from different races were integrated for the first time, and simmering tensions between them saw prisoners group together along ethnic lines in cliques such as the Black Guerrilla Family or the Mexican Mafia.

Initially, their primary purpose was to offer protection from attack.

They are a criminal syndicate first and the ideology comes second Brian Levin, Center for the Study of Hate and Extremism

"Prisons are hostile environments," says former Texas prison warder and gang expert Terry Pelz. "We lock up a lot of people and we have a lot of racial hostility within prison. That's why these gangs form."

Quickly, however, the Brotherhood branched out into smuggling contraband into jails, which gave it a foothold in the lucrative drug trade.

Although its constitution demands that members must be "genetically of European ancestry" and believe in "the racial purity of the white race", its leaders have proved pragmatic in their dealings with non-white outsiders.

"They are a criminal syndicate first and the ideology comes second," says Levin. "They will work with other criminal syndicates even if they belong to ethnicities they dislike - it even says so in their constitution."

They have also forged alliances with other white supremacist prison gangs such as the Nazi Lowriders and the European Kindred.

Today the Southern Poverty Law Center estimates that the California-led Aryan Brotherhood has some 20,000 members both in and out of custody. In 2011 the FBI said it was active in 16 states as far apart as Washington, Virginia, Tennessee and Indiana.

The ABT was formed after Texas desegregated its jails in 1979, when Texan prisoners independently adopted the same markers and structures as the Brotherhood.

It applied to join the wider organisation, but according to TJ Leyden, a former skinhead turned anti-racism campaigner, "the only way to join was to be brought in by a made member, and there were no made members in Texas", so it remains unaffiliated and autonomous.

Greater emphasis was placed by the ABT on signing up members on the outside who could assist with smuggling or drug deals - a practice frowned upon by the mainline Brotherhood, which considered such recruits more likely to accept plea bargains to avoid jail.

What both factions had in common, however, was a steady intake of new inmates who turned to them for protection from the vicious brutality of life in the US prison system.

According to Leyden, the Nazi iconography functions as a means of ensuring these recruits stay loyal, even after they are released, as the groups' codes insist that membership can only be revoked by death.

"They need the swastika, they need the SS bolts, they need these symbols as a form of control," he says.

"If you have that stuff on your body you are not going into a black cell. And when you get out, employers will see the tattoos and say, 'I'm not giving you a shot.' Staying on the street is short-lived for most of them."

Leyden suggests such gangs may have been welcomed by prison guards because they assumed much of the task of policing inmates for them.

If the ABT were responsible for any of the recent killings of justice officials, it would represent a dramatic change in orientation.

Previously the group had been careful to avoid confrontation with the authorities, and some of those who followed their rise have expressed scepticism about whether they are behind the murders.

"If they are involved in this, it's a major step for them," says James W Marquart, a University of Texas criminologist.

"Typically, they are not involved in this sort of high-stakes activity. They like to keep it on the down-low as much as possible."

Alternatively, it may be that the killings are a desperate acknowledgement that the legal proceedings against them posed a serious threat to the ABT's existence.

Few would doubt, however, that groups like this still have the potential to demonstrate to the outside world their potential for sheer barbaric savagery - a capacity that has long been all too familiar to those on the inside.

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