Assistant District Attorney Mark Hasse was shot to death in a parking lot a block from his office on Jan. 31. No arrests have been made in his death.
Lewis declined to say how the couple died or whether authorities believe their deaths are linked to Hasse's. He wouldn't provide further details. Kaufman County is 33 miles southeast of Dallas.
Kaufman Police Chief Chris Aulbaugh told The Dallas Morning News that the McLellands had been shot in their home and although investigators didn't know if their deaths were related to Hasse's killing, they couldn't discount it.
"It is a shock," Aulbaugh told the newspaper. "It was a shock with Mark Hasse, and now you can just imagine the double shock and until we know what happened, I really can't confirm that it's related but you always have to assume until it's proven otherwise."
Sam Rosander, who lives in the same unincorporated area of Kaufman County as the McLellands, told The Associated Press on Saturday that sheriff's deputies were parked in the district attorney's driveway for about a month after Hasse was killed.
Aulbaugh said recently that the FBI was checking to see if Hasse's killing could be related to the March 19 killing of Colorado Department of Corrections head Tom Clements, who was gunned down after answering the doorbell at his home.
Evan Spencer Ebel, a former Colorado inmate and white supremacist who authorities believe killed Clements and a pizza deliveryman two days earlier, was killed in a March 21 shootout with Texas deputies about 100 miles from Kaufman.
Hasse was chief of the organized crime unit when he was an assistant prosecutor in Dallas County in the 1980s, and he handled similar cases in Kaufman County. After Hasse was killed, McLelland had said Hasse was one of 12 attorneys on his staff, all of whom handle hundreds of cases at a time.
"Anything anybody can think of, we're looking through," McLelland said after Hasse's death.
McLelland graduated from the University of Texas before a 23-year career in the Army, according to the website for the district attorney's office. He later earned his law degree from the Texas Wesleyan School of Law.
He and his wife have two daughters and three sons. One son is a police officer in Dallas.