Robert Mueller, the special counsel investigating possible ties between the Trump election campaign and Russia, is expanding his probe to assume control of a grand jury investigation into former national security adviser Michael Flynn, three sources said.

The move means Mueller's politically charged inquiry will now look into Flynn's paid work as a lobbyist for a Turkish businessman in 2016, in addition to contacts between Russian officials and Flynn and other Trump associates during and after the Nov. 8 presidential election.

Federal prosecutors in Virginia are investigating a deal between Flynn and Turkish businessman Ekim Alptekin as part of a grand jury criminal probe, according to a subpoena seen by Reuters.