Donald Trump at Sunday night's debate against Hillary Clinton. REUTERS/Lucy Nicholson A spokesman for Hillary Clinton's campaign tweeted a strongly worded message to Donald Trump during the second presidential debate on Sunday night.

"Hey, @realDonaldTrump — regarding your claim that Captain Khan would be alive if you were president," tweeted Jesse Lehrich, Clinton's foreign policy spokesman. "Go f--- yourself."

The Republican presidential nominee had said during the debate that Army Capt. Humayun Khan, the Muslim soldier killed in Iraq whose parents sparred with Trump this summer after appearing at the Democratic National Convention, would still be alive if Trump had been president because he wouldn't have approved the war in Iraq.

Trump has faced heavy criticism during the election cycle for demeaning Khan's parents.

Lehrich later apologized for his earlier tweet.

"I want to apologize for the clearly inappropriate nature and language of this personal tweet," he tweeted. "Sorry all."

Lehrich then sent this statement to Business Insider: "Needless to say, this was an inappropriate personal tweet that should in no way be taken as a reflection on our campaign. It was a crude visceral reaction, and it was my mistake and my mistake alone."