Attorney General Jeff Sessions 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 (Iowa), Bob Goodlatte (Va.) and Trey Gowdy (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.

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 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 (R-S.C.) to to look into the alleged surveillance abuses within the department.

His decision is likely to not sit well with President Trump, 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 (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.

Huber has also been looking at whether the FBI should have more thoroughly probed Hillary Clinton'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.