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 own chief of staff, Matthew Whitaker.
The president has repeatedly criticised Mr Sessions, a former Republican senator who was the first senator to support Mr Trump's 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.
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.