A police officer in Texas who was being criminally prosecuted for shooting deadan unarmed black man has persuaded a judge to throw out the charge against him by arguing that he enjoyed immunity under the US constitution.

Charles Kleinert, who killed Larry Jackson Jr while serving as a City of Austin police officer, will no longer face a manslaughter trial after a federal judge ruled on Thursday that Kleinert had protection from state charges because he also worked for a federal taskforce.

Judge Lee Yeakel ruled that Kleinert was shielded by the supremacy clause of the constitution when he shot Jackson in the neck during a struggle at the end of a pursuit in July 2013. Kleinert, 51, was indicted for manslaughter by a grand jury last year. He claimed that he fired his pistol accidentally and had intended instead to strike Jackson with the weapon.

Yeakel said the Texas courts had no jurisdiction over Kleinert because he had been investigating an unrelated bank robbery for his federal task force when he began to chase Jackson, and because he believed his actions against the 32-year-old “were no more than was necessary and proper”.

The decision was described as a “complete outrage” by Adam Loewy, an attorney for Jackson’s family, who said he planned to respond by pleading with the US Department of Justice to bring an unlikely federal prosecution against Kleinert.

“This is a legal technicality that will allow a killer to walk free,” Loewy said on Thursday evening. “It is one of the most horrendous moments in the history of civil rights in this country.”