Bartholomew Granger, 42, was previously charged with aggravated sexual assault on a child but at his 2012 trial, allegedly attempted to shoot a trial witness and instead killed Minnie Ray Sebolt, 79, the Beaumont (Texas) Enterprise reported Monday.
The first witness Monday was Criminal District Judge John Stevens, who presided over Granger's original trial and described the scene on the first floor of the Jefferson County Courthouse as chaos.
He said he recognized another woman shot during the incident as Claudia Jackson, a witness in Granger's sexual assault trial.
Jackson and Granger have a daughter together, the newspaper said.
The charge against Granger was elevated to capital murder because Sebolt was killed during the commission of another crime, retaliation against a witness, the prosecution said.
Granger's trial is being conducted in Galveston, about 100 miles from Beayumont, to avoid having jurors walk past the crime scene, the newspaper said, adding Judge Bob Wortham is presiding because other criminal court district judges, including Stevens, were witnesses to the shooting.