Rep. Devin Nunes. J. Scott Applewhite/AP

The founders of the opposition-research firm that produced the dossier alleging ties between President Donald Trump's campaign team and Russia met behind closed doors with House Intel Committee staff on Wednesday and asserted their constitutional privileges not to testify.

The founders of Fusion GPS — Glenn Simpson, Thomas Catan, and Peter Fritsch — were required to appear before the committee by its chairman, Republican Rep. Devin Nunes, who had subpoenaed them earlier this month.

Fusion's counsel, Josh Levy, wrote a 17-page letter to Nunes earlier this week urging him not to force Simpson, Catan, and Fritsch to appear before the committee, because if they did they would have no choice but to assert their constitutional privileges not to testify.

"We cannot in good conscience do anything but advise our clients to stand on their constitutional privileges, the attorney work product doctrine and contractual obligations," Levy wrote.

Nunes required them to appear anyway, prompting Levy to release a blistering statement accusing Nunes — who stepped aside from the committee's Russia investigation in April but still has subpoena power — of abusing his power as chairman.

“No American should have to experience today’s indignity," Levy wrote. "No American should be required to appear before Congress simply to invoke his constitutional privileges. But that is what Chairman Nunes did today with our clients at Fusion GPS, breaking with the practice of his committee in this investigation. The committee has not imposed this requirement on any other witness, including the president’s men."

He added that the "disparate treatment and abuse of power" by Nunes was "unethical, according to the DC Bar rules."

A representative for Fusion GPS confirmed that the cofounders met behind closed doors with the House Intel Committee staff on Wednesday and asserted their privileges not to testify. A representative for Nunes did not immediately respond to a request for comment.

Nunes stepped aside from the committee's Russia investigation in early April following his decision to brief Trump and the press on classified intelligence without telling his fellow committee members. But he quickly began conducting his own investigation into "unmaskings" by the Obama administration and the credibility of the dossier.

Republican Rep. Mike Conaway, who is now heading the committee's investigation, reportedly approved Nunes' subpoenas to Fusion GPS, but House Intelligence Democrats have indicated that they were not consulted.

Simpson, one of Fusion's founders, testified for 10 hours before the Senate Judiciary Committee in August. The firm has also agreed to cooperate with the Senate Intelligence Committee, according to Levy.