The House Intelligence Committee subpoenaed former national security adviser Michael Flynn and former Trump campaign official Rick Gates as part of its reinvigorated investigation into Russian interference in the 2016 election and beyond.

Both men cooperated with special counsel Robert Mueller's Russia investigation and now House Intelligence Chairman Adam Schiff seeks their testimony and documents.

“As part of our oversight work, the House Intelligence Committee is continuing to examine the deep counterintelligence concerns raised in Special Counsel Mueller’s report, and that requires speaking directly with the fact witnesses. Both Michael Flynn and Rick Gates were critical witnesses for Special Counsel Mueller’s investigation, but so far have refused to cooperate fully with Congress," the California Democrat said in a statement.

“That’s simply unacceptable," he added. "The American people, and the Congress, deserve to hear directly from these two critical witnesses. We hope these witnesses come to recognize their cooperation as being with the United States, not merely the Department of Justice.”

Schiff sent letters on Thursday to both Flynn and Gates, setting a June 26 deadline for materials and a July 10 deadline for testimony.

Flynn pleaded guilty in December 2017 for lying to the FBI about his conversations with then-Russian ambassador to the U.S. Sergey Kislyak. He was originally scheduled to be sentenced in December 2018, but that has been postponed by the judge until Flynn has finished cooperating with Mueller’s inquiry.

Gates was indicted in October 2017 as part of Mueller’s investigation alongside his business associate and former Trump campaign chairman Paul Manafort for tax evasion, bank fraud, and failure to register as a foreign agent, among other crimes.

Mueller completed his investigation this year. His team did not find sufficient evidence to establish criminal conspiracy between the Trump campaign and the Kremlin.

The House Intelligence Committee conducted a Russia inquiry of its own under a GOP majority, and in 2018 finished up and found no collusion. But Democrats complained that the effort wrapped up prematurely, and once they took the majority this year Schiff reignited the investigation.