The House Intelligence Committee has invited current and former heads of intelligence and law enforcement agencies to testify next month for its probe into Russian meddling in the 2016 presidential election even as the panel's chairman's credibility has been damaged for alleged bias in the very same investigation.

The committee announced Friday that it had sent out two letters on Thursday, one inviting FBI Director James Comey and National Security Agency Director Admiral Mike Rogers to appear at a closed hearing on May 2; the other inviting former CIA Director John Brennan, Director of National Intelligence James Clapper and former Deputy Attorney General Sally Yates to an open hearing also on May 2.

The committee's investigation, one of several concurrent probes being conducted by Congress and the government, remains mired in controversy. Rep. Devin Nunes, R-Calif., has stayed on as chairman despite criticism from both Republicans and Democrats for his decision to share intelligence information he received on the unmasking of Trump campaign advisers through incidental surveillance with the president before informing members of his committee. Several lawmakers, including the committee's ranking member, Rep. Adam Schiff, D-Calif., have called on Nunes to recuse himself from the case.

Democrats including Schiff have been pushing for Yate to testify ever since Nunes canceled hearings she was to appear in last month.

Yates briefly served as acting attorney general before being fired by Trump in January for not defending his travel ban.

According to a Washington Post report, the Trump administration stepped in to block Yates' testimony after she briefed Trump's White House counsel about former national security adviser Mike Flynn lying to administration concerning his communications with a Russian envoy and was in a position to share potentially damaging information about the Trump administration. The White House denies that it sought to prevent her testimony.

Both Comey and Rogers testified before the House intelligence panel in March, during which Comey confirmed that the FBI is conducting a probe on possible collusion between the Trump campaign and the Russians.

A U.S. intelligence report prepared by the FBI, CIA and NSA in January concluded that Russian government operatives sought to influence and undermine the 2016 election in Trump's favor.