Ukrainian President Volodymyr Zelensky on Sunday sacked his chief prosecutor and the head of the country’s security agency in the largest government shakeup since the start of Russia’s invasion nearly five months ago.

Zelensky said he was firing Prosecutor General Iryna Venediktova and security chief Ivan Bakanov amid a high number of cases of suspected treason by Ukrainian law enforcement officials.

“Today, I made the decision of relieving of their duties the prosecutor general and the head of Ukraine’s security service,” Zelensky said in his daily address to the nation.

Zelensky said over 650 cases of suspected treason and aiding and abetting Russia by Ukrainian security officials are currently being investigated, including 60 cases of officials who have remained in territories occupied by Russia and are working against Ukraine.

“Such a great number of crimes against the foundations of national security and the connections established between Ukrainian law enforcement officials and Russian special services pose very serious questions to the relevant leaders,” Zelensky said.

“Each such question will be answered.”

Venediktova had led Ukraine’s investigation into alleged atrocities committed by invading Russian forces in the town of Bucha outside Kyiv, which caused international outrage.