President Volodymyr Zelensky of Ukraine fired his country’s prosecutor general and the leader of its domestic intelligence agency on Sunday, the most significant shake-up in his government since the start of the full-scale Russian invasion in February.

The dismissals of the prosecutor general, Iryna Venediktova, and Ivan Bakanov, the leader of the Security Service of Ukraine — and a childhood friend of the President — were announced in brief decrees. In a televised speech later on Sunday night, Zelensky said he was responding to a large number of treason investigations opened into employees of law enforcement agencies, including the prosecutor general’s office and the domestic security agency.

On Monday, Zelensky promoted Bakanov’s deputy, Vasyl Malyuk, as the acting head of the security service. Zelensky’s office ordered that Venediktova, who had assumed a prominent and very public role in Ukraine’s efforts to prosecute Russian war crimes and atrocities, be removed from her position. Another decree did the same for Bakanov, citing “Article 47 of the Disciplinary Statute of the Armed Forces of Ukraine.” That pertains to “failure to perform service duties, which led to human casualties or other grave consequences”. A total of 651 cases of high treason had been opened against law enforcement personnel, Zelensky said in his speech.

“Such an array of crimes against the foundations of the national security of the state and the connections detected between the employees of the security forces of Ukraine and the special services of Russia pose very serious questions to the relevant leadership,” Zelensky added.