Sunday night’s contentious presidential debate was especially icy on Twitter, where a Hillary Clinton spokesman responded to comments Republican nominee Donald Trump made about Capt. Humayun S.M. Khan, who was killed in Iraq in 2004.

During the debate, Democratic nominee Hillary Clinton brought up how Trump attacked Khan’s family, who are Muslim, after they spoke about their son’s military service at the Democratic National Convention in July.

Trump responded to Clinton by saying that Khan would still be alive if he were president. Clinton’s foreign policy spokesman had his own response for Trump.

hey, @realDonaldTrump – regarding your claim that Captain Khan would be alive if you were president:



go fuck yourself.#debate — Jesse Lehrich (@JesseLehrich) October 10, 2016

He then apologized for the remarks.

I want to apologize for the clearly inappropriate nature and language of this personal tweet. Sorry all. — Jesse Lehrich (@JesseLehrich) October 10, 2016

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.