Appearing on "CBS This Morning," former FBI assistant director of public affairs John Miller, now a CBS News correspondent, said that the killings of two Texas prosecutors in broad daylight is almost unprecedented in the U.S., and while investigators have few leads so far, a white supremacist gang might be linked to the attacks.

"This is nearly unprecedented, especially on this level of a line prosecutor and then an elected county prosecutor," he said. "This is the kind of thing that they're used to in Colombia and now in Mexico, not in Texas."

Miller added that while police have "not much" to go on, authorities suspect a white supremacist gang might be coordinating the assassinations, in retaliation to a series of prosecutions filed last year against members of the Aryan Brotherhood of Texas.

"There was a threat put out in December against law enforcement who worked on that case, and that is certainly the thing [investigators] looked at first," he said. "But they've been looking at that since January and nothing has come up to the fore with informant information, taps or anything else to point to that, so they're keeping an open mind and looking at everything else too."

The first attack, on prosecutor Mark Hasse, happened at the end of January as Hasse was walking from the Kaufman County courthouse to his vehicle. Police believe two men wearing body armor approached Hasse and shot him multiple times before speeding off. No shell casings were found at the scene.

"That means they either used revolvers, or in a professional move, they picked up the shell casings before they left," Miller said.

In the killings of Kaufman County District Attorney Mike McLelland and wife Cynthia, which happened over the weekend, Miller said the assailants clearly did surveillance of McLelland's home before the attack.

"They entered the home, he and his wife are getting ready for bed, they have an assault-type weapon and there are shell casings everywhere," he explained. "She shot once, I believe, and he shot numerous times. But you don't have that witness who sees anything."

This video is from "CBS This Morning," aired Monday, April 1, 2013.