German federal prosecutors have said the man who drove a vehicle into crowds near Berlin’s Pride celebration on the weekend left a video on his phone pledging allegiance to the Islamic State group. The suspect, 21-year-old German citizen Abdul Ballout, was shot dead by police on Sunday after the attack in Tiergarten park near the Brandenburg Gate late on Saturday, which killed a Polish woman in her 60s and injured 29 people. In the video, the speaker’s face was masked, but authorities assume it shows Ballout; the date of the recording has not been disclosed.

The attack has ignited an acrimonious debate over why Ballout was at liberty. In May, he had been convicted by the Tiergarten district court of preparing a serious act of violence endangering the state and sentenced to one year and ten months’ youth custody, suspended on probation. The general prosecutor’s office, which had demanded a significantly higher penalty, immediately lodged an appeal. Critics have questioned whether juvenile criminal law – generally aimed at milder sanctions – should have been applied in the case of a man known to the authorities as an Islamist.

Berlin’s most senior judicial figures have pushed back forcefully against media criticism. In an unusual joint statement, the president of the Kammergericht, Andrea Diekmann, and general prosecutor Margarete Koppers insisted that the May ruling “was arrived at on the basis and within the framework of the applicable law”. They accused parts of the public debate of “little knowledge of the facts” and warned that “such undifferentiated evaluation, which does not place clarification at its centre, calls the independence of the judiciary – and thus a pillar of the rule of law – into question”.

Germany’s interior minister announced a security overhaul on Monday, though details have not been made public. The federal prosecutor’s investigation continues into how the attack was prepared, whether Ballout had accomplices and what information the authorities possessed beforehand. Diekmann and Koppers said only the ongoing inquiry would determine “whether mistakes were made in the case of Abdul B.”.