An assistant district attorney was shot and killed Thursday morning near the North Texas courthouse where he worked, and authorities said they were searching through his cases to try to find clues about why he may have been targeted.

Mark Hasse, 57, had exited his vehicle in the parking lot behind the Kaufman County Courthouse annex and was walking toward the building when a masked gunman shot him multiple times just before 9 a.m., Kaufman County authorities said. Hasse was taken away in an ambulance, but it's unclear if he died at the hospital or en route.

Investigators were talking to witnesses and had some leads but had not arrested anyone as of Thursday afternoon, Kaufman Police Chief Chris Aulbaugh said. He urged the public to come forward with tips. The suspect or suspects were believed to have fled in a brown or silver older model Ford Taurus. Officials didn't immediately indicate any motive for the shooting in Kaufman, located about 33 miles southeast of Dallas.

"I hope the people that did this are watching because we're very confident that 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," District Attorney Mike McLelland said at a news conference Thursday afternoon.

He said his office, the county and state had suffered a "devastating loss" and called Hasse a spectacular prosecutor who would not be easily replaced. He said Hasse, who worked in a variety of areas such as organized crime, knew the dangers of his job but readily accepted them.

Doug Lowe, longtime district attorney in nearby Anderson County, said Hasse's death is disturbing for all who prosecute crimes in Texas.

"We are a tight group of people, and my heart bleeds for his family and his office," Lowe said. "This reminds us all that we deal with some very, very bad people."

Lowe said he keeps a pistol in his office but plans to start taking it with him.

"This is pretty scary," he said. "I may be packing heat for a while."

Local law enforcement officials decried the shooting as an attack on the criminal justice system.

"We understand that we may come into contact with violent people, but this is the next level," Kaufman County Sheriff David Byrnes said at a news conference.

Hasse, who previously worked as an assistant prosecutor in Dallas County, had worked in Kaufman County for three years. McLelland said Hasse worked hard and took work home with him at night and on weekends. He called him the office "storyteller."

Investigators gathered in a parking lot adjacent to the annex where the prosecutor was shot. Yellow tape and law enforcement vehicles blocked the area from spectators. Aulbaugh said a $20,000 reward was being offered.

"It's going to take a long time to get over this," said Wayne Gent, an attorney whose law office is on the courthouse square. "And the thing is — everybody's vulnerable."

Gent said he had a security system installed at the courthouse when he served as Kaufman County judge but that no system could prevent a shooting that occurs outside.

"How do you protect against that?" he asked.

Carolyn Mikeal, who works at an insurance agency on the square, said the shooting is a stunning development for the usually quiet community.

"It's just a shock," she said. "A small county like this — it's the last thing you expect to happen."