WASHINGTON — Despite pleas from aides, allies, a Republican congressional leader and even his family, President Trump refused to call off the January attack on the Capitol, instead “pouring gasoline on the fire” by aggressively tweeting his false claims of a stolen election and celebrating his crowd of supporters as “very special,” the House investigating committee showed Thursday night.

The next day, he declared anew, “I don’t want to say the election is over.” That was in a previously unaired outtake of an address to the nation he was to give, shown at the prime-time hearing of the committee.

The panel documented how for some 187 minutes, from the time Mr. Trump left a rally stage sending his supporters to the Capitol to the time he ultimately appeared in the Rose Garden video that day, nothing could compel the defeated president to act. Instead, he watched the violence unfold on TV.

“President Trump didn’t fail to act,” said Representative Adam Kinzinger, a Republican Trump critic. “He chose not to act.”

After months of work and weeks of hearings, the prime-time session started the way the committee began — laying blame for the deadly attack on Mr. Trump himself for summoning the mob to Washington and sending them to Capitol Hill.

The defeated president turned his supporters’ “love of country into a weapon,” said the panel’s Republican vice chairwoman, Representative Elizabeth Cheney of Wyoming.

Far from finishing its work after Thursday’s hearing, probably the last of the summer, the panel will start up again in September as more witnesses and information emerge. Ms. Cheney said “the dam has begun to break” on revealing what happened that fateful day, at the White House as well as in the violence at the Capitol.

“Donald Trump made a purposeful choice to violate his oath of office,” Ms. Cheney declared.

“Every American must consider this: Can a president who is willing to make the choices Donald Trump made during the violence of Jan. 6 ever be trusted in any position of authority in our great nation?” she asked.

Mr. Trump, who is considering another White House run, dismissed the committee as a “Kangaroo court,” and name-called the panel and witnesses for “many lies and misrepresentations.”

Plunging into its second prime-time hearing on the Capitol attack, the committee aimed to show a “minute by minute” accounting of Mr. Trump’s actions with new testimony, including from two White House aides, never-before-heard security radio transmissions of Secret Service officers fearing for their lives and behind-the-scenes discussions at the White House.

With the Capitol siege raging, Mr. Trump was “giving the green light” to his supporters by tweeting condemnation of Vice President Mike Pence’s refusal to go along with his plan to stop the certification of President Biden’s victory, a former White House aide told the committee.

Two aides resigned on the spot.

“I thought that January 6 2021, was one of the darkest days in our nation’s history,” Sarah Matthews told the panel. “And President Trump was treating it as a celebratory occasion. So it just further cemented my decision to resign.”

The committee played audio of General Mark Milley, chairman of the Joint Chiefs of Staff, reacting with surprise to the president’s inaction during the attack.

“You’re the commander-in-chief. You’ve got an assault going on on the Capitol of the United States of America. And there’s Nothing? No call? Nothing, Zero?” he said.

At the Capitol, the mob was chanting “Hang Mike Pence,” testified the former deputy national security adviser, Matt Pottinger, as Mr. Trump tweeted his condemnation of his vice president.

Mr. Pottinger, testifying Thursday, said that when he saw Mr. Trump’s tweet he immediately decided to resign, as did Ms. Matthews, who said she was a lifelong Republican but could not go along with what was going on. She was the witness who called the tweet “a green light” and “pouring gasoline on the fire.”

Meanwhile, recordings of Secret Service radio transmissions revealed agents at the Capitol trying to whisk Pence to safety amid the mayhem and asking for messages to be relayed telling their own families goodbye.

The panel showed previously unseen testimony from the president’s son, Donald Trump, Jr., with a text message to his father’s chief of staff, Mark Meadows, urging the president to call off the mob.

Mr. Trump’s son-in-law, Jared Kushner, also testified in a recorded video of a “scared” GOP leader, Congressman Kevin McCarthy, calling him for help.

And in a gripping moment, the panel showed Trump refusing to deliver a speech the next day declaring the election was over, despite his daughter, Ivanka Trump, heard off camera, encouraging him to read the script.