President Donald Trump on Monday said he has fired Federal Reserve Governor Lisa Cook, according to a letter addressed to her posted on his social media — the first instance of a president firing a central bank governor in the central bank’s 111-year history.

Federal Reserve Governor Lisa Cook doubled down on her vow not to resign from the Fed, after President Trump moved on Monday to fire the board member over mortgage fraud allegations. In a statement, Cook argued the president has “no authority” to remove her from her post.

President Donald Trump has fired Federal Reserve Governor Lisa Cook, a sharp escalation in his battle to exert greater control over what has long been considered an institution independent from day-to-day politics.