The announcement, which was expected, comes after criticism of the Justice Department’s handling of the investigation, particularly on how the public was notified.
The email controversy a major issue on the 2016 campaign trail. A message left with Clinton’s office was not immediately returned.
The White House said Thursday it wasn’t involved in the decision to investigate.
“Decisions that are made by inspectors general across the administration are independent, and this administration has assiduously protected the independence of inspectors general,” White House Press Secretary Josh Earnest said at a briefing.