WASHINGTON — The House Jan. 6 committee is set to unveil “surprising” details including evidence from Donald Trump’s Secret Service about the 2021 attack on the U.S. Capitol in what is likely to be its last public hearing before the November midterm elections.

The hearing Thursday afternoon, the 10th public session by the panel, is expected to delve into Trump’s “state of mind” and the central role the defeated president played in the multipart effort to overturn the election, according to a committee aide who discussed the plans on condition of anonymity.

The committee is starting to sum up its findings: Trump, after losing the 2020 presidential election, launched an unprecedented attempt to stop Congress from certifying Joe Biden’s victory. The result was the deadly mob siege of the Capitol.

“The mob was led by some extremist groups — they plotted in advance what they were going to do,” Rep. Zoe Lofgren, D-Calif., a committee member, told CNN. “And those individuals were known to people in the Trump orbit.”

Chairman Bennie Thompson, D-Miss., is poised to gavel in Thursday’s session at an otherwise empty Capitol complex, with most lawmakers at home campaigning for reelection. Several people who were among the thousands around the Capitol on Jan. 6 are now running for congressional office, some with Trump’s backing.

The session will serve as a closing argument by the panel’s two Republican lawmakers, Liz Cheney of Wyoming and Adam Kinzinger of Illinois, who have essentially been shunned by Trump and their party and will not be returning in the new Congress. Cheney lost her primary election and Kinzinger decided not to run.

Another committee member, Rep. Elaine Luria, D-Va., a retired Naval commander, is in a tough reelection bid against state Sen. Jen Kiggans, a former Navy helicopter pilot.

Unlike past hearings, this one is not expected to feature live witnesses, though the panel is expected to share information from its recent interviews — including testimony from Ginni Thomas, the conservative activist and wife of Supreme Court Justice Clarence Thomas. She was in contact with the White House during the run-up to Jan. 6.

Fresh information about the movements of then-Vice President Mike Pence, who was presiding over the joint session of Congress on Jan. 6 and was rushed to safety, is also expected, according to a person familiar with the committee’s planning who was not authorized to discuss it publicly and requested anonymity.

For weeks the panel has been in talks with the U.S. Secret Service after issuing a subpoena to produce missing text messages from that day. Former White House aide Cassidy Hutchinson described being told by a White House aide about Trump angrily lunging at the driver of his presidential SUV and demanding to be taken from his rally to the Capitol as the mob formed on Jan. 6.

Some in the Secret Service have disputed Hutchinson’s account of the events, but it is unclear if the missing texts that the agency has said were deleted during a technology upgrade will ever be recovered. The hearing is expected to reveal fresh details from a massive trove of documents and other evidence provided by the Secret Service.

The committee plans to show new video footage it received from the Secret Service of the rally on the White House Ellipse. Trump spoke there before encouraging his armed supporters to march to the Capitol and “fight like hell.”

The hearing also will include new documentary footage captured from the day of the attack.

The Secret Service has turned over 1.5 million pages of documents and surveillance video to the committee, according to agency spokesman Anthony Guglielmi.

Lofgren said that as she learned the information being presented Thursday she found it “pretty surprising.”

The committee, having conducted more than 1,500 interviews and obtained countless documents, has produced a sweeping probe of Trump’s activities from his defeat in the November election to the Capitol attack.

“He has used this big lie to destabilize our democracy,” said Lofgren, who was a young House staff member during the Richard Nixon impeachment inquiry in 1974. “When did that idea occur to him and what did he know while he was doing that?”

This week’s hearing is expected to be the final investigative presentation from lawmakers before the midterm elections. But staff members say the investigation continues.

The Jan. 6 committee has been meeting for more than a year, set up by the House after Republican senators blocked the formation of an outside panel similar to the 9/11 commission set up after the 2001 terrorist attacks. Even after the launch of its high-profile public hearings last summer, the Jan. 6 committee continued to gather evidence and interviews.

Under committee rules, the Jan. 6 panel is expected to produce a report of its findings, due after the election, likely in December. The committee will dissolve 30 days after publication of that report, and with the new Congress in January.

House Republicans are expected to drop the Jan. 6 probe and turn to other investigations if they win control after midterm elections, primarily focusing on Biden, his family and his administration.

At least five people died in the Jan. 6 attack and its aftermath, including a Trump supporter shot and killed by Capitol Police.

Police engaged in often bloody, hand-to-hand combat, as Trump’s supporters pushed past barricades, stormed the Capitol and roamed the halls, sending lawmakers fleeing for safety and temporarily disrupting the joint session of Congress certifying Biden’s election.

More than 850 people have been charged by the Justice Department in the Capitol attack, some receiving lengthy prison sentences for their roles. Several leaders and associates of the extremist Oath Keepers and Proud Boys have been charged with sedition.

Trump faces various state and federal investigations over his actions in the election and its aftermath.