The House Intelligence Committee subpoenaed former national security adviser Michael Flynn and former Trump deputy campaign manager Rick Gates for documents and testimony as the panel escalates its own post-Mueller probe into Russian meddling during the 2016 presidential election.

Committee Chairman Adam Schiff, D-Calif., announced the subpoenas on Thursday, stating the importance of “speaking directly with fact witnesses.”

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“Both Michael Flynn and Rick Gates were critical witnesses for Special Counsel Robert Mueller’s investigation, but so far have refused to cooperate fully with Congress,” Schiff said in a statement. “That’s simply unacceptable. The American people, and the Congress deserve to hear directly from these two critical witnesses.”

When reached by Fox News, Gates said he “didn’t know” about the subpoena and would get in touch with his lawyers.

Flynn’s new attorney, former federal prosecutor Sidney Powell, had no comment.

Schiff, in his letter to Gates accompanying the subpoena, wrote: “While the Committee understands that your cooperation agreement with the Department of Justice only requires you to testify for the Department, the Committee is disappointed that you do not view your cooperation more broadly as an obligation to assist the United States of America, and not merely the Department of Justice.”

In a similar sentiment, Schiff wrote to Flynn that his cooperation before the committee could “further underscore to your sentencing judge that ‘you’ve done everything you possibly can for the United States of America,’ not just the Department of Justice.”

Both Flynn and Gates were charged as part of Mueller’s probe, but have both had their sentencing hearings delayed for months.

Flynn pleaded guilty to providing false statements to the FBI during a January 2017 interview about his communications with former Russian ambassador Sergey Kislyak in December 2016. His plea was part of a deal to cooperate with then-Special Counsel Robert Mueller’s team and federal prosecutors.

Gates was a close associate of former Trump campaign chairman Paul Manafort, and also did work for the Trump campaign in 2016. He pleaded guilty last year to one count of conspiracy against the U.S. and one count of making false statements to FBI agents. Gates also entered a plea deal with prosecutors to cooperate in the case against Manafort, which led Mueller to dismiss nearly two-dozen tax and bank fraud charges against Gates. Gates had intimate knowledge of Manafort’s years of political consulting work in Ukraine.

Schiff’s move to subpoena Flynn and Gates comes as his panel, along with other top House committees, ramp up their Trump-focused investigations. Schiff’s panel is hosting a series of hearings focused on the “counterintelligence implications” of Mueller’s report, with plans to host “fact witnesses, national security experts” and others connected to the Mueller investigation. Other Democrat-led panels are pursuing a similar approach, with some moving to hold Trump figures in contempt for not complying with subpoenas.

Republicans, in turn, have accused Democrats of investigation overkill, with GOP Rep. Devin Nunes accusing Schiff at a hearing on Wednesday of putting on a "grotesque spectacle."

The hearings come after Mueller made a rare public appearance last week to mark the conclusion of his investigation and the closing of his office. He maintained that there was “not sufficient evidence to charge a conspiracy” with regard to whether members of the Trump campaign coordinated with the Russian government during the 2016 presidential election.

But Mueller left open the question of whether the president obstructed justice.

“If we had had confidence that the president clearly did not commit a crime, we would have said that,” Mueller said. “We did not determine whether the president did commit a crime.”

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Mueller explained longstanding Justice Department policy, which states a sitting president cannot be charged with a crime, and thus said “charging the president was not an option we could consider.”

Meanwhile, the House Judiciary Committee, led by Chairman Jerrold Nadler, D-N.Y., is probing whether the president obstructed justice.

Fox News' Alex Pappas, Ronn Blitzer and Gregg Re contributed to this report.