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Several Democrats are expressing outrage after letters surfaced that they said suggested the White House tried to prevent former acting Attorney General Sally Yates from testifying before a House Intelligence Committee in its probe of connections between the Trump campaign and Russian officials.

"It sounds like an effort to try and muzzle persons who have information and the ability to shed light on this investigation," Rep. Jackie Speier, D-California, a member of the House Intelligence Committee, told MSNBC on Tuesday.

White House officials deny such claims and say the administration never sought to block Yates from testifying.

White House press secretary Sean Spicer called the assertion "100 percent false" during Tuesday's press briefing.

"I hope she testifies," Spicer said adding he is "looking forward to it".

House Intelligence chairman Rep. Devin Nunes, R-Califonia, also denied the claims and added that the committee still planned to question her at some point.

“Neither Chairman Nunes nor any Intelligence Committee staff members had any communication with the White House whatsoever about Sally Yates testifying to the Committee,” Nunes said in a statement. “The only person the Committee has spoken to about her appearing before the Committee has been her lawyer. The Committee asked her to testify on our own accord and we still intend to have her speak to us.”

Yates was fired by President Trump in January after refusing to defend the administration’s executive order barring travel from seven Muslim-majority countries.

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Yates, former CIA Director John Brennan, and former Director of National Intelligence James Clapper — all former Obama-administration appointees — were set to appear for public hearings on Tuesday on Russian interference in the 2016 election.

However, Nunes abruptly cancelled that session, over the objections of Democrats. Instead, FBI Director James Comey and National Security Agency Director Adm. Michael Rogers were asked to return to the Hill to testify in a closed session on that date.

Ultimately, that meeting, too, was cancelled due to a scheduling conflict and will be rescheduled.

Nunes has also scrapped the rest of the committee's meetings this week.

The House is currently investigating Russian influence on the election, but also possible ties between Russia and Trump associates, and the previous administration's use of surveillance, which President Trump asserted without evidence and against the testimony of top intelligence and justice officials had been used illegally to target Trump Tower.

Correspondences between Yates’ counsel David O’Neil, the committee’s chairman and ranking member, and the Justice Department obtained by NBC News and first published by the Washington Post show Yates requesting permission from the administration to speak at a hearing on Russian interference.

Read the letters

In a March 23 letter to Acting Assistant Attorney General Samuel Ramer, O’Neil wrote that Yates would not discuss classified information but that the guidance they received went further and was “overbroad, incorrect, and inconsistent with the department’s historical approach to the congressional testimony of current and former officials.”

Associate Deputy Attorney General Scott Schools responded that because Yates was seeking to testify on conversations she had with the White House Office of Counsel, the material would be covered by executive privilege and referred her to the White House.

He added she “need not obtain separate consent” from the Justice Department.

In a letter to White House counsel Donald McGahn on March 24, O’Neil said any guidance barring Yates from discussing conversations about an unnamed senior White House official would be “untenable” and that the relevant topics had already been discussed publicly by top officials.

Rep. Adam Schiff, D-California, the ranking Democrat on the House Intelligence Committee, said in a statement that the cancelled hearing would have enabled members to question Yates about the lead-up to the president’s decision to fire former National Security Adviser Michael Flynn over conversations with Russian Ambassador Sergey Kislyak that he failed to disclose to Vice President Mike Pence. He speculated that the White House may have been involved in the decision to block the meeting.

“We are aware that former AG Yates intended to speak on these matters, and sought permission to testify from the White House,” Schiff said. “Whether the White House's desire to avoid a public claim of executive privilege to keep her from providing the full truth on what happened contributed to the decision to cancel today's hearing, we do not know.”

Spicer denied that claim and called the Washington Post story “false.”

“The White House has taken no action to prevent Sally Yates from testifying and the Department of Justice specifically told her that it would not stop her and to suggest otherwise is completely irresponsible,” Spicer told NBC News.

White House spokeswoman Sarah Huckabee Sanders noted that O’Neil’s last letter to McGahn, dated March 24, said they would conclude the administration had not asserted executive privilege over the topics in question if they heard no response by Monday, March 27 at 10 AM.

“We took NO action, therefore not blocking her,” Sanders said.

The back-and-forth is the latest sign of tension between the White House, Nunes, and Democrats, some of whom have called for Nunes to step down from the investigation after he revealed he had met with a source at the White House before telling reporters that intelligence agencies may have incidentally monitored the president and his aides’ communications.