U.S. Attorney General William Barr told federal prosecutors on Monday to look into "substantial" allegations of irregularities in last week's election, prompting the top lawyer overseeing voter fraud investigations to resign in protest.
Barr sent his letter after days of attacks on the integrity of the election by President Donald Trump and Republican allies, who have alleged without evidence that there was widespread voter fraud.
Trump has not conceded the election to Democrat Joe Biden who on Saturday secured more than the 270 votes in the Electoral College needed to win the presidency.