Three US Democratic senators sent a letter to President Barack Obama on Thursday, asking him to compel CIA Director John Brennan to admit that his agency's search of Senate Intelligence Committee staffers' computers two years ago was improper and promise that it will never happen again.

"In January 2014, CIA personnel conducted an unauthorized, unprecedented search of Senate committee files, including the emails and other files of Senate staff investigating the CIA's use of torture," says the letter, which was signed by Senators Ron Wyden, Martin Heinrich, and Mazie Hirono, who are each members of the Senate Intelligence Committee. "The CIA Inspector General stated in a July 2014 report that this search involved 'improper agency access to [Senate Intelligence Committee] files.' A review board selected by CIA Director Brennan concluded in December 2014 that this CIA search 'resulted in inappropriate access to [Senate Intelligence Committee] work product'."

The missive underscores long-running tensions between the committee and the CIA. It was sent to Obama in anticipation of Brennan's first public testimony before the committee in two years, which is scheduled for Tuesday afternoon. The senators sent a nearly identical letter to the CIA last year, but never received a response. They are now saying that it is crucial that the president intercede and "address this matter directly."

"We believe that it is necessary for you to ensure that senior officials in your administration recognize the importance of adhering to the rule of law," the lawmakers wrote to Obama. "We ask that you instruct Director Brennan to acknowledge that the CIA's unauthorized search of Senate files was improper and will not be repeated."

The Intelligence Committee's damning report on the CIA's torture program — which concluded that the CIA's use of so-called "enhanced interrogation techniques" such as waterboarding was not effective and did not produce "unique" and "valuable" intelligence — was released in December 2014. But the infighting over the report's integrity and the allegations of CIA spying and hacking has continued to play out publicly for two years, resulting in chilled relations between the agency and its congressional overseers.

In a dramatic 45-minute floor speech on March 11, 2014, then-Intelligence Committee chairwoman Dianne Feinstein accused the CIA of spying on the committee, intimidating her staffers, and attempting to block the release of the committee's report about the CIA's torture program. The dispute led to a near-constitutional crisis between the agency and the committee, with Feinstein insisting that the search of senate staffers' computers encroached upon separation of powers between Congress and the executive branch.Brennan, meanwhile, vehemently rejected the charges.

A lengthy investigative report by VICE News based on hundreds of pages of CIA documents that were obtained in response to a Freedom of Information Act lawsuit revealed last August that Feinstein's explosive charges had merit. The CIA appears to have improperly searched the computers of Senate staffers and violated a 2009 agreement between the agency and the committee, which established that the committee would be able to independently conduct its work without any interference from the CIA.

A CIA accountability review board assigned to review the incident later exonerated a handful of CIA personnel who were found to be responsible for the search and blamed the breach on a series of miscommunications.

Publicly, the CIA attempted to explain that the search was warranted because senate staffers had gained access to a "privileged" set of internal CIA documents about the torture program known as the Panetta Review, named after former CIA Director Leon Panetta. But internal CIA documents turned over to VICE News showed that a senate staffer obtained the Panetta Review through a misconfigured Google search tool set up by a CIA contractor who had been paid $40 million by the agency.

Moreover, the documents turned over to VICE News included a July 28, 2014 letter from Brennan that was addressed to Feinstein and Saxby Chambliss, who was then the ranking Republican on the Intelligence Committee, in which he apologized to them and admitted that the CIA's penetration of the computer network used by committee staffers reviewing the agency's torture program was improper.

But Brennan never signed or sent the letter, which the CIA said had been disclosed to VICE News by mistake. Instead, he sent a different message that did not contain an apology or an admission that the search was improper. He did, however, verbally apologize to Feinstein during a private meeting with her and Chambliss.

The letter sent by the three senators to Obama on Thursday did not refer to Brennan's unsent letter. They said his refusal to acknowledge that the search was improper or commit that it would never happen again "is wholly unacceptable behavior in a democracy."

"You have repeatedly spoken about the need for the American public to have confidence that senior intelligence officials respect US laws and the Constitution," they wrote. "Director Brennan's refusal to acknowledge that unauthorized clandestine searches of Senate files are improper has clearly undermined that confidence."

The White House did not respond to a request for comment. The CIA declined to comment.