(CNN) -- Mark Hasse "had an absolute passion for putting away bad guys, and he enjoyed nothing better," his boss said.
Now investigators are looking into whether that passion led to Hasse being gunned down outside his office in Kaufman, Texas, where he was an assistant district attorney. Hasse was killed in the parking lot of the Kaufman County courthouse Thursday morning, shot several times after "a very small, very short confrontation," Police Chief Chris Aulbaugh said.
"It was apparent that he was not expecting to have anything happen," Aulbaugh said. "He was on his way to his office."
Investigators are looking into whether Hasse's killing was retribution for any of the prosecutions he led, Aulbaugh said, "but we can't say we're confident that that's where it came from."
District Attorney Mike McLelland called Hasse "a stellar prosecutor" who knew that threats were part of the job, and he vowed to put away the "scum" who killed his assistant.
"I hope that the people that did this are watching, because we're very confident that we're going to find you," McLelland said at a news conference Thursday afternoon. "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."
Hasse was one of 13 prosecuting attorneys in Kaufman County, each of whom handled between 380 and 390 cases, McLelland said.
"He was a lot of fun. He was the premier storyteller of the office. He had lots of stories to tell," including surviving the crash of a World War II aircraft that left him in a coma, "caved in half his skull" and cost him his sense of smell.
"It was five kinds of miracles he survived that," McLelland said.
There have been no arrests, and Aulbaugh and other officials who addressed reporters Thursday afternoon appealed to the public for help. The local CrimeStoppers group and a Kaufman businessman have put up a total of $20,000 in reward money for information leading to a conviction, Aulbaugh said.
The Texas Rangers, the FBI and agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives have joined the manhunt, and "we are in the process of running down many leads right now," he said.
Kaufman County sheriff's spokeswoman Pat Laney said investigators are looking for an older-model, four-door sedan, either brown or silver, but it wasn't clear whether there were one or two suspects Thursday afternoon, Laney said.
"We have officers going over all of this case," she said. "Nothing has been identified yet that could have prompted this assault."
CNN's Michael Martinez, Rich Phillips, Marlena Baldacci, Nick Valencia and John Branch contributed to this report.