A sheriff's official lowers the Kaufman County flag to half-staff on March 31. AP Photo/Mike Fuentes The murder a Texas district attorney and his wife months after an assistant DA was gunned down in broad daylight is virtually unprecedented in the United States.

Former SWAT officer Glenn McGovern, an expert on these types of assassinations, shed some light on the recent Texas slayings in an interview with NPR. He's never seen such brazen assassinations back to back.

"I have only seen this type of targeted attack, where you hit two members within a short period of time within the same organization, in attacks in Sicily and Colombia and Mexico. It, to my knowledge, has never happened here in the U.S.," McGovern told NPR.

Kaufman County DA Mike McLelland and his wife, Cynthia, were killed at home on Saturday two months after assistant DA Mark Hasse was shot dead in broad daylight outside a courthouse.

Their killers are at large, but officials believe the Aryan Brotherhood of Texas may be the culprit since the DA's office was part of a huge crackdown on the white supremacist prison gang.

While such back-to-back assassinations are rare, McGovern says targeted killings of prosecutors, judges, and senior law enforcement officials are on the rise.

There have been 15 of these types of attacks so far in this decade — a significant and "unprecedented" increase from previous decades, according to McGovern. (Three years into the 1980s there were only two targeted attacks of senior law enforcement.)

The attacks on the McLellands and Hasse were particularly unnerving, though. They'd been warned the Aryan Brotherhood of Texas might be seeking retribution, and Hasse carried a gun to work every single day.

McLelland carried a gun everywhere and seemed to be on high alert.

“I’m ahead of everybody else because, basically, I’m a soldier,” McLelland, an Army veteran, told the AP just two weeks before he was killed.

If two officials on high alert can be slayed, what's to stop disgruntled defendants from ordering hits on any prosecutor who file charges against them?

McGovern actually said he doesn't think a "white supremacist prison gang" is behind the Texas prosecutors' slayings because the killers managed to get away so easily.

"To me, that takes some skill, to get away cleanly as well. Just, on the face of it, it shows me that you're dealing with somebody who has some training," McGovern told NPR.

It's also possible a prison gang has become so powerful it can hire highly skilled hit men to intimidate the people who can put its members behind bars. That is a truly terrifying possibility.