Authorities investigating shooting deaths of Kaufman County District Attorney Mike McLelland and his wife, Cynthia

(CBS News) An intense investigation is under way in Texas where Kaufman County District Attorney Mike McLelland and his wife were found shot to death Saturday, just two months after one of McClelland's prosecutors was shot and killed near his courthouse office.

The District Attorney's Office will be closed on Monday and security will be tight for other government workers in Kaufman, Texas in the wake of the killings. The courthouse will be open but officials are promising visibly tighter security. Multiple agencies, including the Texas Rangers and the FBI are now beginning their search to hunt down the killer or killers.

Investigators swarmed the Forney, Texas home of 63-year-old McLelland and his 65-year-old wife Cynthia.

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Sources tell CBS News an unknown killer or killers used an assault rifle to commit the murders as early as Friday night.

Kaufman County Sheriff David Byrnes had little to say but this: "It's unnerving for the law enforcement community. It's unnerving to the community at large."

The murders come just two months after another Kaufman County prosecutor, Assistant District Attorney Mark Hasse, was gunned down in a parking lot just a block from the courthouse where he worked.

That same day, McLelland stood firm at a news conference after his prosecutor was murdered, sending an impassioned warning to those who killed Mark Hasse. "We're very confident that we're going to find you, we're going to pull you out of the hole you're in, we're going to bring you back and let the people of Kaufman County prosecute you to the fullest extent of the law," he said then.

McLelland -- an Army veteran who served for 23 years -- is said to have carried a gun everywhere after the Hasse murder. In an interview with CBS News, he seemed determined. Asked what part of the ordeal hurts the most, he replied, "The fact that I can't reach out and grab somebody and do something about it right now because I want them really bad."

The murders have unnerved Kaufman residents. "You can't believe that this thing would happen. Happened the first time but now it's happened twice," resident Bryant Martin said.

The FBI, the Texas Rangers and other law enforcement agencies have been looking into possible connections to white supremacist groups who had been targeted by McLelland's office.

For Anna Werner's full report, watch the video in the player above.

CBS News senior correspondent John Miller, a former FBI assistant director, called the case "nearly unprecedented."

"Especially on this level of a prosecutor, aligned prosecutor and then the elected county prosecutor, this is the kind of thing that they're used to in Colombia and now in Mexico, not in Texas," he said.

Law enforcement investigating the case don't have much to go on, according to Miller.

"In the case of Mark Hasse, who was the deputy district attorney who was shot first, he is leaving the District Attorney's Office, crosses the street, walks to the DMV headquarters where he has his car parked. So someone has done surveillance - where does he work and where does he park, where is the best place to get him? There you've got one, possibly two men, wearing black tactical vests, masks, weapons, they open fire on him, kill him by his car, they leave in a silver vehicle, no shell casings were recovered. That means they either used revolvers or in a professional move, they picked up the shell casings before they left.

Watch John Miller's full "CTM" analysis in the video below.

"Now let's move up to this case. In the case of D.A. McLelland ... they clearly watched his home, they've seen the police car that was there has been dropped, they enter the home. He and his wife are getting ready for bed, they have assault-type weapon and there's shell casings everywhere. She's shot once and he's shot numerous times but you don't have the witness who sees anything," Miller said.

Asked if investigators have any idea who is doing this, Miller said they've had ideas from the beginning, but none of those ideas are "panning out readily." He explained, "That doesn't mean they won't come around back and get a clue."

Miller said, "Let's start off with the red herring or the first idea that comes in which is both men were involved in a case involving ABT, Aryan Brotherhood of Texas, all-white prison gang. This was a major sweep involving a bunch of district attorneys offices. There was a threat put out in December against law enforcement who worked on that case, and that is certainly the thing they looked at first, but they've been looking at that since January, and nothing has really come up to the fore with an informant, information taps or anything like that, to point to that. So they are keeping an open mind and looking at everything else, too."