Whitaker was the Chief of Staff to Sessions prior to Sessions' departure.
Sessions said in a letter to Trump that he resigned at the president's request. He will serve our Country well ...
Trump repeatedly has said over the past year that he regretted making Sessions attorney general.
Mr. Trump added that a "permanent replacement" for Sessions "will be nominated at a later date".
"I don't have an attorney general".
Mueller will now answer to Whitaker, who will oversee the investigation.
Trump's own description of his frustrations with Sessions, who was the first Republican senator to endorse Trump's candidacy, began shortly after Sessions took office.
Trump and Sessions had a falling out after the attorney general recused himself from Mueller's investigation.
Trump announced in a tweet that he would be temporarily replaced by his chief of staff. While the current Acting AG, Deputy Attorney General Rod Rosenstein, has stood by Mueller and insisted he would not fire him without good cause, Whitaker may feel differently.
Matthew Whitaker will serve as acting attorney general.
"How do you take a job and then recuse yourself?" said Trump.
Sessions's recusal stems from the fact he was an active supporter of Trump's 2016 campaign.
"Mueller has come up to a red line in the Russian Federation 2016 election-meddling investigation that he is dangerously close to crossing", Whitaker wrote. Whitaker has criticized special counsel Robert Mueller's investigation into potential coordination between the president's Republican campaign and Russian Federation.
Despite this warning, Romero was far from upset about Sessions' exit, calling him "the worst attorney general in modern American history".
That led to a series of personal attacks by the president who repeatedly called Mr. Sessions names ranging from "an idiot" to "disgraceful".
Sessions usually does not respond to the President's criticism, but he has at times pushed back.
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