The U.S. House of Representatives Intelligence Committee has subpoenaed two witnesses of special counsel Robert Mueller's Russia inquiry as part of its own investigation into alleged Russian interference in the 2016 U.S. presidential election, the panel announced Thursday.

This combination of file pictures created on June 13, 2019 shows former Trump campaign official Rick Gates (L) leaving Federal Court on December 11, 2017 in Washington, DC; and former US National Security Advisor General Michael Flynn arriving for his sentencing hearing at US District Court in Washington, DC. [File photo: AFP/Saul Loeb, Brendan Smialowski]

The committee subpoenaed former White House National Security Advisor Michael Flynn and former Donald Trump campaign aide Rick Gates for "documents and testimony," according to a statement.

Chairman Adam Schiff said that the committee is examining "deep counterintelligence concerns" raised in Mueller's investigative report, a redacted version of which was made public and to Congress in April.

Flynn and Gates were "critical witnesses" for Mueller's investigation, but so far have refused to cooperate fully with Congress, the California Democrat said.

A retired U.S. Army lieutenant general, Flynn admitted lying to federal investigators about his conversations with the Russian ambassador to the United States and cooperated with Mueller for the Russia investigation.

Gates pleaded guilty to conspiracy and false statement charges related to Ukrainian lobbying and political consulting work he did with Paul Manafort, former Trump campaign chair, who has been sentenced to more than seven years in prison.

Mueller concluded his 22-month investigation in March by submitting a report to Attorney General William Barr.

The report stated that there was no evidence that Trump's campaign conspired with the Russian government during the 2016 U.S. presidential election but didn't conclude if the president had obstructed justice.

Instead, Mueller recounted 10 episodes involving Trump and discussed potential legal theories for connecting these actions to elements of an obstruction offense.

It was the Department of Justice that concluded that the Mueller did not have "sufficient" evidence to support a charge in the obstruction case, a move that has drawn scrutiny from Democrats.