Republican presidential nominee Donald Trump told Hillary Clinton that if he becomes president, he will instruct the attorney general to hire a special prosecutor to investigate her. (The Washington Post)

Donald Trump said during Sunday's presidential debate that, if elected, he would appoint a special prosecutor to look into Hillary Clinton's use of a private email server while she was secretary of state.

"I didn’t think I’d say this and I’m going to say it and I hate to say it. … If I win, I am going to instruct my attorney general to get a special prosecutor to look into your situation because there has never been so many lies, so much deception," Trump said.

In July, FBI Director James B. Comey said that his agency would not recommend criminal charges against Clinton, but that she and her staff were "very careless" in their handling of classified information.

Trump has made Clinton's use of email a central issue on the campaign trail.

"We're going to get a special prosecutor and we're going to look into it," he said.

Clinton did not directly respond, but again apologized for her use of a private server.

"That was a mistake and I take responsibility," she said.

Clinton said there is "no evidence" that anyone hacked the server and no evidence that classified material ended up in the wrong hands.