With little solid information, speculation on who is behind the killings has included a white supremacist gang targeted by Texas and federal authorities last year, drug cartels and someone with a personal grudge against the slain prosecutors.
The white supremacist angle gained traction in part because McLelland, in an interview with The Associated Press before his death, speculated that the Aryan Brotherhood of Texas could have been behind Hasse's slaying.
"We put some real dents in the Aryan Brotherhood around here in the past year," McLelland told the news agency.
McLelland's office was one of numerous Texas and federal agencies involved in a multi-year investigation that led to the indictment last year of 34 alleged members of group, including four of its senior leaders, on racketeering charges.
At the time, Assistant U.S. Attorney Lanny A. Breuer called the indictment a "devastating blow" to the organization, which he said used threats and violence -- including murder -- against those who violate its rules or pose a threat to the enterprise.
The FBI describes the group as a "whites only," prison-based gang operating since at least the 1980s.
While authorities have not said whether they have linked the deaths of Hasse and McLelland, or the involvement of white supremacists, Texas law enforcement agencies did warn shortly after the November 2012 indictment that there was "credible information" the group was planning to retaliate.
Frank Meeink, a former white supremacist skinhead, said the group could well be behind the killings.
"The indictment that happened a year ago to this group really hurt the Aryan Brotherhood of Texas," he told CNN's "Starting Point" on Tuesday. "This isn't an ideology battle here. This is financial."
On the other hand, in the AP interview, McLelland said Hasse wasn't involved in the Aryan Brotherhood investigation. And a gang expert said killing public officials doesn't fit with the group's profile.
"This would be a giant leap for them to kill public officials," said Terry Pelz, a former Texas prison warden and expert on the Aryan Brotherhood.
He thinks drug cartels concerned about disruptions in the methamphetamine supply are more likely culprits.
Speculation has also extended to whether the shootings have any connection to the March 19 death of Colorado prisons chief Tom Clements, who was gunned down after answering the door at his home.
While authorities have offered no suggestion the crimes are linked, Evan Ebel -- the man suspected of killing Clements -- was once a member of a white supremacist group and died in a shootout with sheriff's deputies in northern Texas on March 21.
Pete Schulte, a friend of the McLellands and a criminal defense attorney who has worked in the county, said other lawyers and public servants are nervous.
"Having that type of environment going on where people who are just doing their jobs (and) getting assassinated -- this is what this is, elected officials getting assassinated -- and that is sending a chill through the (legal) community and the community in general," he said.
He speculated that the killings were "personal."
"If this was a case that somebody was trying to change, they would have been going after witnesses and not the prosecuting attorney," Schulte said.
Michael Burns, McLelland's former law partner, said prosecutors from several counties have exchanged theories about what happened.
"But frankly, none of us know."
"We're used to hearing this sort of thing happening in Colombia or even Mexico. We're not used to hearing about judicial officials targeted in the United States. It's hard to say whether this is a local phenomenon that involves only one issue locally there, or whether this is the beginning of a trend. As a prosecutor, I can just tell you, we can't ignore it," Burns said.
"We're looking out the peephole when the doorbell rings now, where we maybe we weren't before," he said.
Filling a void