Cassidy Hutchinson, who has provided the committee with important testimony behind closed doors, is expected to testify publicly on Tuesday afternoon.

The House committee investigating the Jan. 6 attack on the Capitol is expected to hear public testimony on Tuesday from Cassidy Hutchinson, a former aide to Mark Meadows, the final chief of staff for President Donald J. Trump, according to people familiar with the matter.

The committee on Monday abruptly scheduled a hearing for Tuesday afternoon to present what the panel called “recently obtained evidence.” But it did not disclose the nature of the evidence or say who would be testifying, touching off a wave of speculation.

The hearing is scheduled for 1 p.m. on Capitol Hill, according to a news release issued by the committee. Ms. Hutchinson’s planned testimony was reported earlier by Punchbowl News.

Ms. Hutchinson has provided the panel with some of its biggest revelations to date, all made during videotaped closed-door testimony, portions of which have been shown or alluded to in previous hearings.

She is said to have been present when Mr. Meadows described hearing Mr. Trump react approvingly to chants by his supporters to hang Vice President Mike Pence. 

And she testified that a half-dozen Republican lawmakers who led the efforts in Congress to overturn the election sought pardons after the riot.

It was not immediately clear what a short-notice hearing to hear from Ms. Hutchinson might reveal, given that she has already testified three times behind closed doors after receiving a subpoena and the committee has featured clips of her testimony in previous hearings.

The announcement — and its sudden and secretive nature — gave way to a day of guesswork about what the panel might have learned, or whose cooperation it may have secured, to warrant upending a carefully choreographed hearing schedule during a week when members had left Washington to spend time in their districts around the country.

“BETTER BE A BIG DEAL,” John W. Dean, the White House counsel under President Richard M. Nixon known for his role in the Watergate scandal, wrote on Twitter.

“There was only one surprise witness during the Senate Watergate Committee hearings. On July 16, 1973 an unannounced witness appeared: Alex Butterfield, who testified to Nixon’s secret taping system — forever changing history!”

The Jan. 6 panel has held a series of hearings this month laying out the findings of its nearly yearlong investigation, but it had not been scheduled to have any additional sessions until July.