(CNN) -- Mike McLelland came to the law late in life.
He was already around 40 when he started law school at Texas Wesleyan University in Fort Worth, drawn in by his work with the mentally ill, classmate and one-time law partner Michael Burns said. Becoming a district attorney "was what he always wanted to do," Burns said.
McLelland was killed over the weekend, shot to death along with his wife, Cynthia. Friends found the couple dead Saturday evening in their home in Forney, on the eastern outskirts of Dallas.
He was elected in 2010 to the post of district attorney in Kaufman County, where one of his assistant prosecutors was gunned down outside the courthouse in January. Now his own slaying is at the heart of a Texas-size whodunit that has left local, state and federal investigators scrambling for answers.
"They were the consummate good people," Burns said. "We kidded Mike because he had no identifiable vices, and we all had vices. We either drank too much or smoked too much or chased women. But Mike had no identifiable vices, and Cynthia was solid gold."
Another colleague, Anderson County District Attorney Doug Lowe, called McLelland "a big bear of a guy, never met a stranger."
"He said what he meant, and I'm sorry this tragedy followed him," Lowe told CNN's The Situation Room.
Both McLellands had master's degrees in psychology, said Burns, now the district attorney in Palo Pinto County, on the opposite side of the sprawling Dallas-Fort Worth metroplex. They had met in the field and were dating while Mike attended law school, he said.
Their marriage was the second for both; they had five children between them, all grown now, Burns said.
The 63-year-old McLelland grew up in Wortham, where his parents had a ranch. He joined the Army after attending the University of Texas and still held a major's commission in the reserves when Iraq invaded Kuwait in 1990, sparking the Persian Gulf war. Burns said McLelland was called up shortly after finishing law school and trained soldiers in psychological warfare during the 1991 conflict.
Burns, who had been a police officer, and McLelland were older than the average student at Texas Wesleyan and soon became study partners.
"He decided that he wanted to go into the law particularly to be a prosecutor, but those opportunities don't come along that often," Burns said. "So when we graduated, we went down to Corpus Christi, down on the coast, and opened a law office and practiced criminal defense work."
Drawing on his earlier training, McLelland took on a lot of court-appointed work for mentally ill defendants, Burns said. Cynthia was a gourmet cook, and "she used to bring some of the most fantastic lunches to the office when we were there," he said.
Eventually, both moved back to north Texas, where McLelland set up a law practice in Kaufman County. He had been a psychologist for the Texas Department of Mental Health and Mental Retardation before going to law school and became a mental health judge after getting his law degree; his wife got a job at the state mental hospital in Terrell and was still practicing there when she was killed, Burns said.
Burns was elected to his office in 2006. When McLelland decided to run for Kaufman County's chief prosecutor's post four years later, his friend came over to "get the lay of the land about what it was like to be an elected DA"
Pete Shulte, another friend of McLelland's, told CNN's Starting Point that lawyering was a collegial business in the county of about 100,000.
"Everybody knew each other here. It was a great place to practice law," Shulte said. "Everybody liked the district attorney's office. There just wasn't a lot of activity out here. So the biggest shock out here this morning is why, in Kaufman, Texas, are we having an assistant DA get killed and an elected DA. It's really sending some shock waves through the community."
McLelland won a three-way race in the 2010 Republican primary and ran unopposed in the general election. Burns said he and McLelland kept in touch "here and there" until January, when McLelland's assistant district attorney, Mark Hasse, was shot to death in a still-unsolved case.
"He was bound and determined to find out who had done that to Mark, and Mike was fearless," Burns said. He was like, 'They better come prepared, because there'll be a fight.' "
After Hasse's killing, McLelland publicly vowed to bring the "scum" who killed his assistant to justice.
"We're going to pull you out of whatever 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.
But in mid-February, Burns said McLelland told him, "My greatest fear is that it was one person, acting alone, who doesn't drink," eliminating the chance that a liquored-up perpetrator would ever confess.
"That's vintage McLelland right there."
Since the killings, Burns 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," Burns said. "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."
"We're looking out the peephole when the doorbell rings now, where we maybe we weren't before," he added.
CNN's Steve Almasy contributed to this report.
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