Public relations firms with ties to Flynn, Manafort, subpoenaed by Mueller

— -- Six public relations firms with ties to former Trump campaign chairman Paul Manafort or former National Security Advisor Michael Flynn have received subpoenas from special counsel Robert Mueller, ABC News has learned from sources with knowledge of the situation.

The subpoenas relate to the probe Mueller is leading that is investigating Russian interference into the 2016 presidential election and potential Russian collusion with Trump campaign associates.

Manafort has faced scrutiny over past lobbying work done on behalf of the government of Ukraine, ultimately requiring him to register as a foreign agent in July. That month, the FBI executed a search warrant on Manafort's Virginia home.

Flynn was forced to resign as National Security Advisor in February after misleading Trump administration officials about the nature of his pre-inauguration discussions with then-Russian ambassador to the United States Sergey Kislyak. This past spring, federal prosecutors began using a grand jury in Alexandria, Virginia to issue subpoenas to collect evidence on Flynn and any potential ties to Russian officials.

Mueller's prosecutors are now using a Washington, D.C. grand jury for matters related to their investigation.