Mark Hasse, 57, was shot in what officials said was a well-planned attack, The Dallas Morning News reported Thursday.
The attackers, who witnesses said were dressed in black, escaped in a sedan after shooting Hasse as he emerged from his car on his way to work.
Hasse was pronounced dead at a local hospital.
At least one of the shooters wore a mask and a bulletproof vest, witnesses said.
After Hasse was shot, one of the attackers fired into the air, apparently to scare people away.
An employee of an auto body shop across the street said he heard five shots in quick succession and then two more.
County officials said they were reviewing hundreds of Hasse's past cases for any possible suspects in the shooting. Authorities are investigating the possibility of a link between the shooting and the Aryan Brotherhood prison gang, which the prosecutor's office has been pursuing.
Focusing on the brotherhood could be premature, officials said, because Hasse was not currently handling any cases involving the organization.
"We can't say it was a hit," Kaufman Police Chief Chris Aulbaugh said. "We're pursuing all possibilities."
County offices and all schools in the district were locked down after the shooting. All attorneys in the prosecutor's office were placed under guard.