In the space of two months Texas has been rocked by three apparently targeted killings, which have left police and the FBI hunting for suspects and local prosecutors fearing for their safety.

On 31 January Kaufman County's assistant district attorney, Mark Hasse, was shot five times in a parking lot near his courtroom office. Two months later district attorney Mike McLelland and his wife, Cynthia, were found dead in their home. Reports suggested 14 shots had been fired.

The deaths have thrown a spotlight on the Aryan Brotherhood of Texas, a white supremacist group identified by one anti-racist organisation as the most violent extremist group in the US.

The Aryan Brotherhood of Texas started as a prison gang in the northern part of the Texas in the 1980s, but now has over 2,000 members both in and out of jail, engaged in a wide variety of criminal activities. In November, 34 members of the ABT were indicted on federal racketeering charges. Among the agencies involved in that prosecution was Kaufman County's district attorney's office.

Assistant District Attorney Mark Hasse, 57, who was shot and killed Jan. 31, 2013, in Kaufman, Texas. Photograph: Handout/AP

Hasse, 57, (pictured) was killed the day the FBI issued a press release stating that two ABT members were pleading guilty to the charges. Among the agencies listed as helping achieve that conviction was Kaufman County.

After Hasse's death McLelland gave a defiant press conference where he vowed to personally bring the "scum" responsible to justice. But in the weeks before he and his wife were killed he had taken to carrying a gun at all times, and told reporters he now took extra care when opening the door at his home. In an interview with the Associated Press not long before he died, he raised the possibility that the ABT had killed Hasse.

The Anti-Defamation League, an organisation which monitors hate groups in the US, has identified the ABT as "the most violent extremist group in the United States". It says the gang was responsible for "at least" 29 murders in the US since 2000.

As the ABT transitioned from a race-related prison gang to an organised crime syndicate, its leaders began to sacrifice Aryan ideology for business purposes. Mark Potok, from the Southern Poverty Law center, which monitors hate groups, told Democracy Now on Monday that while the racism within the ABT is and other white supremacist groups certainly exists, it can be "an overlay" for what are "fundamentally criminal enterprises".

"So these groups, while they have all kinds of, quote-unquote, 'Aryan ideology' and so on, are very quick to make alliances with, say, the Mexican Mafia, the Black Guerrilla Family – in other words, non-white prison gangs – especially if it will help them in the running of methamphetamine and heroin and drugs like that," Potok said.

The pragmatic approach to crime and relations with other criminal outfits is not to say the ABT has lost touch with its roots, however.

"They are horribly racist and horribly antisemitic," Clark said. "In addition to their criminal activity they have an ideology that is filled with hate. And they certainly are incredibly violent."

While ABT certainly have form when it comes to violent crime, Clark said that does not necessarily mean they are responsible for the deaths in Kaufman County. A criminal group like this one would not normally seek retribution from officials, she said.

"While plots and plans to target law enforcement are not outside the realm of possibility, and wouldn't be surprising by these groups, it is unusual based on past history," she said. "As of right now nothing has been substantiated which connects them. That doesn't mean there won't be later, but there's not evidence right now."

Police too, have been reluctant to publicly link the ABT to Hasse and the McLellands' deaths. Behind the scenes, however, there have been revealing moments. An email from an official involved in investigating Hasse's death said: "The focus of our investigation involves the Aryan Brotherhood of Texas (ABT) being responsible for the murder of the ADA."

Texas congressman Ted Poe, a former judge and prosecutor, said on Tuesday that he believed "the Aryan Brotherhood" would be behind the killings, although he did not specifically identify the ABT. Perhaps most damningly, the Texas Department of Public Safety had issued a statewide bulletin warning in December that the ABT was "actively planning retaliation against law enforcement officials" who helped secure the 34 indictments in Houston. Kaufman County's district attorney's office was among the agencies on the case.

"High ranking members … are involved in issuing orders to inflict 'mass casualties or death' to law enforcement officials who were involved in cases where Aryan Brotherhood of Texas are facing life sentences or the death penalty," the bulletin said.

Internal violence

The Aryan Brotherhood is believed to have been formed in San Quentin state prison, California, in the 1960s. But Clark said the Aryan Brotherhood of Texas is run autonomously and not linked to the wider group.

Over the past 12 years ABT members have committed multiple "torture murders", according to the Anti-Defamation League, and has been responsible for murders where victims' bodies were "burned, decapitated or otherwise dismembered in order to make identification difficult".

Clark said it was impossible to know how many ABT members were in prison at any one time. "They're in and out," she said. "But whether they're in prison or out of prison, if they're part of the Aryan Brotherhood of Texas, they're part of the group."

Leaving the ABT is not as easy as joining it, Clark says. "It's not when you're in prison you're in, and when you're out you're out. If you are in the group you are in the group – it doesn't matter if you're incarcerated or you're not incarcerated."

For those who do join, the risks are high. Much of the violence carried out by the ABT is internal, and members can become targets if they are perceived as "snitches" or attempting to start factions. "It is almost as dan­ger­ous to be an ABT mem­ber as it is to encounter one," the Anti-Defamation League study says. The league estimates that some 41% of known murders carried out by the ABT from 2000-12 were "internal killings".