Ukrainian President Volodymyr Zelenskyy issued executive orders late on Sunday sacking the head of Ukraine’s powerful domestic security agency, the SBU, and the prosecutor general.

The orders dismissing SBU chief Ivan Bakanov, a childhood friend of Zelenskyy, and Prosecutor General Iryna Venediktova, who led the effort to prosecute Russian war crimes in Ukraine, were published on the president’s official website.

In a separate Telegram post, Zelenskyy said he had fired the top officials because many cases had come to light of members of their agencies collaborating with Russia.

He said 651 treason and collaboration cases had been opened against prosecutorial and law enforcement officials, and that more than 60 officials from Bakanov and Venediktova’s agencies were now working against Ukraine in Russian-occupied territories.

“Such an array of crimes against the foundations of the national security of the state … pose very serious questions to the relevant leaders,” Zelenskyy said.

“Each of these questions will receive a proper answer,” he said.

Zelenskyy appointed Oleksiy Symonenko as the new prosecutor general in a separate executive order that was also published on the president’s site.