Daniel Schlienz, 42, a former boxer, was taken into custody after the shooting Thursday at the Cook County Courthouse in Grand Marais, the Duluth (Minn.) News Tribune reported.
Cook County Attorney Tim Scannell and another victim were hospitalized, officials said, while a third person was treated and released. The other two victims were not identified.
Gary Schlienz said his son "really hated the prosecuting attorney." He said Ginger Berglund, Daniel Schlienz's mother, was with him in the courthouse and tried to wrest the gun from him.
"I don't want to make excuses for him, but they prosecuted him pretty bad. He had no job, no money, nothing," Gary Schlienz said.
Daniel Schlienz entered an Alford plea to sexual assault in 2006, conceding there was enough evidence to convict him without admitting guilt. A year later, he filed a motion to withdraw the plea, and an appeals court ruled in January that he could do so.
A jury returned a guilty verdict Thursday afternoon, the report said. Scannell was the prosecutor at the trial.