The shooting occurred in a parking lot near the Kaufman County Courthouse, WFAA-TV, Dallas/Fort Worth, reported.
Kaufman Police Chief Chris Aulbaugh said an assistant district attorney was shot multiple times about 8:50 a.m. as he walked from his car, The Dallas Morning News reported. The prosecutor, whose identity was not released, died of his injuries, Aulbaugh said.
The victim had been part of an investigation of members of the white supremacist group the Aryan Brotherhood, the newspaper said.
"It's a horrible situation," Kaufman County Judge Bruce Wood told the Morning News. "None of us would have ever expected anything like this to ever happen in our county."
The courthouse was closed for the day after authorities searched the area for suspects.
The Texas Department of Public Safety sent out an alert for troopers to be on lookout for an older model silver Ford Taurus. Witnesses said the suspects were wearing all black and at least one was wearing a tactical vest.
The gunman reportedly fired shots in the air as the two suspects fled.