Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE revealed in a letter to lawmakers Thursday that he had declined to name a second special counsel to investigate allegations of surveillance abuse within the Department of Justice (DOJ), despite pressure from the Republican Party for him to do so.

In his letter to GOP committee chairmen Chuck Grassley Charles (Chuck) Ernest GrassleyGardner signals support for taking up Supreme Court nominee this year Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee McConnell digs in on vow to fill Ginsburg's Supreme Court seat MORE (Iowa), Bob Goodlatte Robert (Bob) William GoodlatteNo documents? Hoping for legalization? Be wary of Joe Biden Press: Trump's final presidential pardon: himself USCIS chief Cuccinelli blames Paul Ryan for immigration inaction MORE (Va.) and Trey Gowdy Harold (Trey) Watson GowdySunday shows preview: Election integrity dominates as Nov. 3 nears Tim Scott invokes Breonna Taylor, George Floyd in Trump convention speech Sunday shows preview: Republicans gear up for national convention, USPS debate continues in Washington MORE (S.C.), Sessions said the appointment of a special counsel only occurs under "the most 'extraordinary circumstances.' "

"To justify such an appointment, the Attorney General would need to conclude that 'the public interest would be served by removing a large degree of responsibility for the matter from the Department of Justice,' " Sessions wrote.

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He said he had named a federal prosecutor in Utah, John Huber, to lead the investigation into Republicans' allegations that the FBI and DOJ abused a surveillance program against a former Trump campaign aide.

"The additional matters raised in your March 6, 2018, letter fall within the scope of his existing mandate, and I am confident that Mr. Huber's review will include a full, complete, and objective evaluation of these matters in a manner that is consistent with the law and the facts," Sessions wrote.

Sessions said that upon completion of the investigation he would receive a recommendation from Huber about whether the allegations merit the need for another special counsel.

The letter comes amid mounting pressure from GOP lawmakers for Sessions to appoint a second counsel as Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE continues his own investigation into possible ties between the Trump campaign and Russia during the 2016 presidential campaign.

Sessions earlier this month said he had tapped a former official "with many years in the Department of Justice" to look into the need for another special counsel, as calls grew from the likes of Grassley, Gowdy and Sen. Lindsey Graham Lindsey Olin GrahamGraham: GOP will confirm Trump's Supreme Court nominee before the election Fox's Napolitano: Supreme Court confirmation hearings will be 'World War III of political battles' Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee MORE (R-S.C.) to to look into the alleged surveillance abuses within the department.

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His decision is likely to not sit well with President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE, who has frequently criticized the attorney general over his decision to recuse himself from the Russia investigation.

In his letter, Sessions also noted that he had directed the department's inspector general, Michael Horowitz, to open a probe into the allegations, an investigation that was formally announced on Wednesday. Trump has also been dismissive of the decision to put the department's watchdog in charge of the case.

"Why is A.G. Jeff Sessions asking the Inspector General to investigate potentially massive FISA abuse," Trump tweeted earlier in March after Sessions announced Horowitz would be looking into the surveillance abuse allegations. "Will take forever, has no prosecutorial power and already late with reports on [former FBI Director James] Comey etc. Isn’t the I.G. an Obama guy? Why not use Justice Department lawyers? DISGRACEFUL!"

The multiple investigations center on allegations from Republicans that the FBI and DOJ abused the Foreign Surveillance Intelligence Act (FISA), which were the subject of a controversial memo authored by House Intelligence Committee Chairman Devin Nunes Devin Gerald NunesSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Sunday shows preview: With less than two months to go, race for the White House heats up Sunday shows preview: Republicans gear up for national convention, USPS debate continues in Washington MORE (R-Calif.).

The GOP lawmakers allege that investigators used information from the "Steele dossier," a controversial file of opposition research compiled by ex-MI6 agent Christopher Steele, in order to obtain a politically motivated warrant for former Trump adviser Carter Page.

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Huber has also been looking at whether the FBI should have more thoroughly probed Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE’s ties to Uranium One, a Russian nuclear energy agency.

Former President Obama appointed Huber to his position in 2015.

Updated at 5:58 p.m.