Washington (CNN) House Intelligence Chairman Adam Schiff declined on Sunday to say whether the House will subpoena former national security adviser John Bolton but said "the truth will come out" in one form or another.

"I don't want to comment to this point on what our plans may or may not be with respect to John Bolton, but I will say this: whether it's before -- in testimony before the House -- or it's in his (forthcoming) book or it's in one form or another, the truth will come out (and) will continue to come out," Schiff said in an interview with CBS when asked if the chamber would subpoena the former Trump administration official.

The comments come two days after the Senate voted to block any witnesses from being called in the trial, thwarting Democrats' efforts to include witnesses such as Bolton and acting White House chief of staff Mick Mulvaney in the proceedings. Calls to include Bolton in the trial increased last week in the wake of revelations from his draft book manuscript that alleged Trump told him US security aid to Ukraine was conditioned on investigations into Democrats.

Trump's attempts to pressure Ukraine to investigate Hunter Biden and Joe Biden, his potential 2020 general election rival, are at the center of the President's impeachment trial. Trump and his allies have repeatedly made unfounded and false claims to allege that the Bidens acted corruptly in Ukraine.

On Friday, new allegations from Bolton's manuscript that Trump directed Bolton to help his personal attorney Rudy Giuliani get in touch with the Ukrainian President in May were reported by The New York Times , and Schiff cited them during Friday's witness debate.

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