An FBI computer analysis response team has joined the investigation.
The team is scouring for digital evidence on computers, phones and other electronics belonging to the McClellands and Hasse, said Katherine Chaumont, a spokeswoman for the FBI's Dallas Field Office.
Such forensic investigations could be used to search for threats that might have been made against them and could help investigators learn more about the victims' lives.
Chaumont would not specify whether the investigators were examining devices belonging to anyone else, such as witnesses or possible suspects.
Investigating local residents
Since Hasse was gunned down near the county courthouse on January 31, authorities have pored through his case files, including public corruption cases, to see if any defendants he tried may have sought vengeance.
Authorities met with Eric Williams, a former justice of the peace who was convicted last year of burglary and theft by a public servant.
Surveillance video showed Williams apparently stealing computer monitors from the county courthouse. He was sentenced to two years' probation.
Saturday night, hours after the McLellands were found dead in their home, investigators met Williams at a local Denny's restaurant, his attorney told CNN Tuesday.
Investigators took swab samples from Williams' hand to test for gun residue, attorney David Sergi said.
The law enforcement official said Williams is "one angle we are looking at," the official said.
Sergi says his client voluntarily cooperated because he has nothing to hide.
Another Kaufman County resident has drawn the interest of investigators.
The man, who has been trying to open a gun range on his property, was involved in a civil dispute with McLelland and the county.
The resident told CNN that FBI agents visited him and asked a few questions, but nothing else materialized.
Speculation about the Aryan Brotherhood of Texas
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 said Hasse wasn't involved in the Aryan Brotherhood of Texas investigation, but his office was one of numerous Texas and federal agencies involved in an investigation that led to the indictment last year of 34 alleged members of the group -- including four of its senior leaders -- on racketeering charges.
Authorities have not said whether they have linked white supremacists to the deaths. But weeks after the indictment, the Texas Department of Public Safety said it had "credible information" the Aryan Brotherhood of Texas was planning to retaliate.
Asked about the group's possible involvement, the governor said it was too soon to link or discount the it.
"I think it's obviously too early to be speculating on whether there is any direct contact, but I think it's wise for us to not overlook any evidence that either may be superficial or otherwise," Perry said on Fox News.
"So they are here, they are active in this state. We know the drug cartels are very, very active in our country now," Perry said.