WASHINGTON — The House committee investigating the January 6 riot has interviewed nearly 1,000 people. But the nine-member panel has yet to talk to the two most prominent players in that day’s events — President Trump and Vice President Pence.

As the investigation winds down and the panel plans a series of hearings in June, members of the committee are debating whether to call the two men, whose conflict over whether to certify Mr. Biden’s 2020 presidential election win was at the center of the attack. Mr. Trump pressured Mr. Pence for days, if not weeks, to use his ceremonial role presiding over the January 6 count to try to block or delay Mr. Biden’s certification. Mr. Pence refused to do so, and rioters who broke into the building that day called for his hanging.

There are reasons to call either or both of them. The committee wants to be as thorough as possible, and critics are sure to pounce if they don’t even try. But some lawmakers on the panel have argued that they’ve obtained all the information they need without Messrs. Trump and Pence.

Nearly a year into their wide-ranging investigation into the worst attack on the Capitol in more than two centuries, the House committee has interviewed hundreds of witnesses and received more than 100,000 pages of documents. Interviews have been conducted out of the public eye in obscure federal office buildings and private Zoom sessions.

The Democratic chairman, Representative Bennie Thompson, said in early April that the committee has been able to validate a lot of the statements attributed to Messrs. Trump and Pence without their testimony. He said at that time there was “no effort on the part of the committee” to call Mr. Pence, though there have been discussions since then about potentially doing so.

Speaking about Mr. Pence, Mr. Thompson said the panel had “initially thought it would be important” to call him, but “there are a lot of things on that day we know — we know the people who tried to get him to change his mind about the count and all of that, so what is it we need?”

A lot of the people they are interviewing, Mr. Thompson added, “are people we didn’t have on the original list.”

The panel, comprised of seven Democrats and two Republicans, has said that the evidence it has compiled is enough to link Mr. Trump to a federal crime.

Much of the evidence the committee has released so far has come from White House aides and staff — including little-known witnesses like a former special assistant in the Trump White House, Cassidy Hutchinson, and Mr. Pence’s chief counsel in the vice president’s office, Greg Jacob. 

The panel also has thousands of texts from Mr. Trump’s final chief of staff, Mark Meadows, and has talked to two of the former president’s children, Ivanka Trump and Donald Trump Jr., who were with their father the day of the attack.

Among hundreds of others, the committee has also interviewed former White House aide Jared Kushner, Ivanka’s husband, former communications director Alyssa Farah and multiple Pence aides, including his chief of staff, Marc Short, and his national security adviser, Keith Kellogg. Former White House press secretaries Kayleigh McEnany and Stephanie Grisham have also appeared, as has former senior policy adviser Stephen Miller.

There are still questions that Messrs. Trump and Pence could answer, including what they talked about the morning of January 6, when Mr. Trump made his final plea for Mr. Pence to overturn the election when he presided over the Electoral College count in Congress. Lawmakers have been able to document most of Mr. Trump’s end of the call but not what Mr. Pence said in response.

In the hours after Messrs. Trump and Pence spoke, the vice president issued a statement saying he did not have the power to object to the counting of electoral votes. But the president did not relent, and went on to publicly pressure Mr. Pence at his massive rally in front of the White House and then on Twitter even after his supporters had broken into the Capitol.

Still, it is unlikely that the two former leaders would speak about the conversation to the committee — and it’s unclear if they would cooperate at all.

While Mr. Pence has yet to comment on the committee’s work, Mr. Trump would certainly be a hostile witness. He has fought the investigation in court, demonized the committee on TV and tried to assert executive privilege over White House papers and any conversations he had with his aides — demands that would certainly apply to his morning call with Mr. Pence.

In addition, calling a former president or vice president to testify in a congressional investigation is a rare, if not unprecedented, move that could face major legal hurdles and backfire politically.

The January 6 committee has given only a glimpse of what it has found, mostly in court filings where excerpts of transcripts have been used.

A recent filing from the committee revealed portions of interviews with Mr. Hutchinson that took place in February and March of this year. That testimony provided new evidence about the involvement of GOP lawmakers in Mr. Trump’s effort to overturn the 2020 election, including a meeting at the White House in which attorneys for the president advised that putting up an alternate slate of electors declaring Mr. Trump the winner was not “legally sound.”

Another court document revealed testimony from Mr. Jacob, who served as Mr. Pence’s chief counsel. In a series of emails, Mr. Jacob repeatedly told lawyer John Eastman, who was working with Mr. Trump, that Mr. Pence could not intervene in his ceremonial role and halt the certification of the electoral votes. Mr. Jacob told Mr. Eastman the legal framework he was putting forward to do just that was “essentially entirely made up.”

Mr. Meadows’ texts have also been revelatory, detailing how people inside Mr. Trump’s orbit pleaded for him to forcefully condemn the attack on the Capitol as it unfolded. The pleas came from Mr. Trump’s children, members of Congress and even Fox News hosts.

“He has to lead now. It has gone too far and gotten out of hand,” Donald Trump Jr. texted Meadows as protesters breached the security perimeter at the Capitol.