Ukrainian President Volodymyr Zelensky has sacked two key figures in Ukraine's security services on Sunday, his cabinet announced.
"Today I made a decision to remove the Prosecutor General from office and to dismiss the Head of the Security Service of Ukraine," Zelensky said in his nightly video address.
The two officials concerned are Prosecutor General Iryna Venediktova and State Security Service (SBU) head Ivan Bakanov, a long-time associate of the president.
Ms Venediktova led the investigation into the alleged atrocities committed at the start of the invasion by Russian forces in the town of Bucha, northwest of Kyiv.
Suspicion of treason
Zelensky said that many people in both departments were suspected of treason. "As of today, 651 criminal proceedings have been initiated for treason and collaboration activities of employees from prosecutor's offices, pretrial investigation bodies, and other law enforcement agencies," he said.
"In particular, more than 60 employees of the prosecutor's office and the Security Service of Ukraine remained in the occupied territory and are working against our state."
Explaining his reasons for removing the officials, Zelensky stressed that "such a large number of crimes against the foundations of national security and the links established between Ukrainian law enforcement officials and Russian special services raise very serious questions for the leaders concerned."
Zelensky added that the former Head of the Main Directorate of the Security Service in Crimea has been arrested due to suspicion of treason.
"Everyone who together with him was part of a criminal group that worked in the interests of the Russian Federation will also be held accountable. It is about the transfer of secret information to the enemy and other facts of cooperation with the Russian special services."
Blamed for Russian success in the south
Senior figures of the SBU in Ukraine's south have been have been blamed for the easy with which Russian troops captured large parts of the south after a week of Russia's invasion.
Zelensky suggested action would be taken moving forward.
"The specific actions and any inaction of each official in the security sector and in law enforcement agencies will be evaluated. The corresponding inspection of law enforcement agencies has already yielded the first results and will be continued."
The Ukrainian President added that the Prime Minister would be mandated to intensify the search in securing a new chief for the National Anticorruption Bureau.