US Attorney General Jeff Sessions has been fired by President Donald Trump.
Mr Trump announced in a tweet that Mr Sessions would be temporarily replaced by his chief of staff.
The president has repeatedly criticised Mr Sessions, a former Republican senator who was among the first to support Mr Trump's presidential candidacy.
"We thank Attorney General Jeff Sessions for his service, and wish him well!" Mr Trump wrote on Wednesday.
In a letter, Mr Sessions said he was resigning at the president's request.
....We thank Attorney General Jeff Sessions for his service, and wish him well! A permanent replacement will be nominated at a later date.— Donald J. Trump (@realDonaldTrump) November 7, 2018
Mr Trump lost trust in his top law enforcement officer after Mr Sessions announced that he would step aside from the Russia investigation last year, and allow his deputy Rod Rosenstein to lead the politically-charged probe.
"Dear Mr President, at your request I am submitting my resignation," Mr Sessions said in his letter.
"Most importantly as my time as attorney general, we have restored and upheld the rule of law," he wrote, while also thanking the Republican president.
Mr Trump has repeatedly pilloried his top law enforcement official since Mr Sessions stepped aside from the Russia investigation in March 2017, allowing his deputy Rod Rosenstein to lead an inquiry that has dogged the White House.
In July 2017 Mr Trump told the New York Times: "Sessions should have never recused himself, and if he was going to recuse himself, he should have told me before he took the job and I would have picked somebody else."
Mr Trump has at various times belittled Mr Sessions as "beleaguered", "VERY weak", and "DISGRACEFUL".