The leaders of the House committee investigating the Jan. 6 attack on the Capitol are promising a vigorous inquiry into a day they have called a threat to American Democracy, which could lead to an unprecedented legal and political showdown over how to force members of Congress to take the witness stand.

Several congressional Republicans have admitted to having some contact with President Donald Trump during the insurrection or in the days leading up to it, making their testimony potentially key to the panel’s stated goal of being “guided solely by the facts.”

The Jan. 6 panel’s chairman, Rep. Bennie G. Thompson (D-Miss.), said in an interview that there is “no reluctance to subpoena” any member of Congress “whose testimony is germane to the mission of the select committee” if they resist cooperating voluntarily.