Original sentence: Former KGB officer Oleg Gordievsky believes that the murders of writers Yuri Shchekochikhin (author of "Slaves of KGB" ), Anna Politkovskaya, and Aleksander Litvinenko show that FSB has returned to the practice of political assassinations  practised in the past by the Thirteenth KGB Department
Edited sentence: Former KGB officer Oleg Gordievsky believes that murders of writers Yuri Shchekochikhin (author of "Slaves of KGB" ), Anna Politkovskaya, and Aleksander Litvinenko show that FSB has returned to the practice of political assassinations  which were conducted in the past by Thirteenth KGB Department