At least 13 senior Russian military officers were detained for questioning after Wagner Group founder Yevgeny Prigozhin’s rebellion, The Wall Street Journal reported on Thursday, citing knowledgeable sources.

According to the newspaper, Russia’s Federal Security Service (FSB) began arresting people just hours after the mutiny began. It also reported that around 15 officers have been removed from service or fired.

One of the officers detained was reportedly General Sergey Surovikin, who previously served as the country’s top commander in Ukraine and has not been seen publicly since the day of the revolt. According to the Wall Street Journal’s sources, Surovikin is in state custody in Moscow. One source said that the general knew of Prigozhin’s mutiny plans but did not participate in them. The sources also said that Prigozhin has not been charged with any crimes and that he will likely be released as soon as Vladimir Putin decides how to respond to the rebellion.