Hasse had feared for his life and carried a gun to work, said a Dallas attorney who described herself as his longtime friend.
Colleen A. Dunbar said she spoke with Hasse on January 24. She said the prosecutor told her he had begun carrying a gun in and out of the county courthouse daily.
"He told me he would use a different exit every day because he was fearful for his life," Dunbar told CNN.
She said that Hasse gave no specifics on why he felt threatened -- only that he did.
McLelland called Hasse "a stellar prosecutor" who knew that threats were part of the job.
He vowed after Hasse's slaying to put away the "scum" who killed his deputy.
"I hope that the people that did this are watching, because we're very confident that we're going to find you," McLelland told reporters.
"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."
Attorney Pete Schulte told CNN affiliate WFAA that public servants are facing a new quandary.
"It's going to have a chilling effect on people who do want to step into those roles and (have to think about whether to) start arming themselves," he said. "I mean, that's the risk that we're going to face now because of this happening."
Schulte told the station that after someone shot through the windows of his Dallas offices in November, he began to carry a gun more often.