Trump calls Clinton 'guilty as hell' and says the FBI was 'very nice to her'
The department's Office of Inspector General said the probe would focus on FBI Director James Comey's public announcement of the department's decision not to bring charges against Clinton for her use of a private server while serving as secretary of state.
It will also examine the subsequent letter to Congress - sent just several days before the general election - announcing the FBI's continued investigation into the server.
"What are Hillary Clinton's people complaining about with respect to the FBI," Trump tweeted Friday morning. "Based on the information they had she should never have been allowed to run - guilty as hell. They were VERY nice to her. She lost because she campaigned in the wrong states - no enthusiasm!"
Trump often called Clinton "guilty as hell" during his campaign over her use of a private email server. But after he won the election, Trump backed off his "lock her up" threats of new investigations.
"I don't want to hurt the Clintons, I really don't," Trump told The New York Times in November. "She went through a lot and suffered greatly in many different ways, and I am not looking to hurt them at all. The campaign was vicious."