The House select committee investigating the Jan. 6 Capitol insurrection begins work Tuesday, hearing from police officers who confronted the deadly chaos. Reminding Americans that the Jan. 6 riot was a horrific attack on democracy — in contrast to the narrative some Republicans have told of a “loving crowd” filled with people behaving like average Washington tourists — is an essential part of the committee’s work.

But, also in contrast to Republican claims, there is much for the select committee to uncover.

Top of the list is precisely what then-President Donald Trump did before, during and after the attack. How did he prepare his speech preceding the insurrection, in which he told the crowd to fight? What did he anticipate his audience’s reaction would be? When did he know the pro-Trump mob was threatening the Capitol? Why did he offer only mild statements long after the danger was clear? Did Trump-affiliated rally organizers coordinate with extremist groups? Answering such questions calls for subpoenaing former White House chief of staff Mark Meadows; Mr. Trump’s daughter Ivanka and her husband, White House senior adviser Jared Kushner; and other White House aides with useful information.

Also relevant is what members of Congress reported to Mr. Trump and other members of his administration as the riot unfolded. House Minority Leader Kevin McCarthy (R-Calif.), who spoke with the president and Mr. Kushner on Jan. 6, must testify, along with any other lawmakers who interacted with the Trump administration in the run-up to, during and after that day. The list includes Sen. Tommy Tuberville (R-Ala.), Rep. Mo Brooks (R-Ala.) and possibly Rep. Jim Jordan (R-Ohio). For that matter, the committee must examine whether any lawmakers themselves maintained connections with or even abetted the rioters.

Investigators should hear from extremist-group leaders at the center of the violence. How did they prepare? What was their goal? The committee should hear also from Justice Department and Capitol Police officials who failed to anticipate the riot. Why did intelligence officials across the government seem unaware of warnings that were all over social media? To what extent did law enforcement discount or ignore warning signs about right-wing extremists because federal and local officers did not want to cross Mr. Trump and other Republicans? Why did the National Guard take so long to arrive?

Finally, the investigation should lead to recommendations to forestall a repeat of such political violence, with a particular focus on how the government monitors domestic extremism. As they conduct their work, the lawmakers on the largely Democratic panel must suppress the urge to make it the partisan exercise that Republicans claim it will be — behaving instead like the fact-finders the nation needs.

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