US Attorney-General Jeff Sessions has resigned, after being asked to by President Donald Trump.
The 71-year-old has been repeatedly criticised by Mr Trump in the past year, after recusing himself from the investigation into whether the Trump campaign in 2016 had help from Russia. Mr Sessions had previously denied meeting Russian officials, but later admitted he had.
After stepping aside from that investigation, his deputy Rod Rosenstein ramped it up by appointing a special counsel, Robert Mueller, with the power to prosecute alleged crimes it uncovers.
"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 told the New York Times shortly afterwards in 2017.
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In a letter, Mr Sessions said he was asked to resign by Mr Trump - effectively being fired.
Mr Trump says Mr Sessions will be replaced in the short-term by his chief of staff, Matthew G Whitaker. A permanent replacement will be "nominated at a later date".
Mr Mueller's probe has indicted several members of Mr Trump's campaign, including aide Rick Gates, policy advisor George Papadopoulos, personal lawyer Michael Cohen, chair Paul Manafort and National Security Adviser Michael Flynn.
Mr Whitaker will assume oversight of the Russia probe, CNBC reported.
"Protecting Mueller and his investigation is paramount," said Democratic Party Senator Chuck Schumer.
"It would create a constitutional crisis if this was a prelude to ending or greatly limiting the Mueller investigation and I hope President Trump and those he listens to will refrain from that."