Two congressional committees on Tuesday plan to hold simultaneous hearings digging into the attack on the U.S. Capitol by at mob of Trump supporters, asking questions of generals and law enforcement leaders about the security failures that helped led to violence and death on Jan. 6.

Representative Carolyn B. Maloney, Democrat of New York and the chairwoman of the House Oversight Committee, said she planned to unveil her committee’s research into the delayed response of the National Guard, showing that Capitol Police and D.C. officials made 12 “urgent requests” for Guard support during the attack and that Army leaders told the National Guard to “stand by” five times as the violence escalated.

“That response took far too long,” Ms. Maloney said in a statement. “This is a shocking failure, and today we intend to get to the bottom of why it happened.”

Beginning at 2 p.m., members of House Oversight and Reform Committee plan to question Christopher Wray, the F.B.I. director; General Charles Flynn, who commands the U.S. Army Pacific; and Lieutenant General Walter Piatt, director of the Army staff.

General Flynn and General Piatt were involved in a key mid-riot call with police leaders in which Army leaders worried about the “optics” of sending in the Guard, according to those involved in the meeting. It is the first time lawmakers will hear testimony from either man.

General Piatt has defended his caution in advising against sending in the National Guard. “The last thing you want to do is throw forces at it where you have no idea where they’re going, and all of a sudden it gets a lot worse,” he told The New York Times in January.

General Flynn is the brother of Michael T. Flynn, the disgraced former national security adviser under President Donald J. Trump who has emerged as one of the ex-president’s biggest promoters of the lie of a stolen election.

Documents obtained by the committee show that beginning at 1:30 p.m. on Jan. 6, top officials at the Defense Department received at least 12 urgent requests for help from the Capitol Police chief, the mayor of Washington, Muriel Bowser, and other officials. But the National Guard did not arrive until 5:20 p.m., more than four hours after the Capitol perimeter had been breached.

The panel will not hear testimony from the acting chief of the Capitol Police, Yogananda D. Pittman, who declined to attend, citing her need to hear testimony at the other committee hearing.

In that hearing, also Tuesday afternoon, the Committee on House Administration plans hear testimony from the Capitol Police inspector general, Michael A. Bolton, and Gretta L. Goodwin, director of Homeland Security and Justice for the U.S. Government Accountability Office.

Mr. Bolton intends to testify about his fourth investigative report into the failures of Jan. 6, which found that the department’s tactical unit did not have access to “adequate training facilities”; did not have adequate policies for securing ballistic helmets and vests (two dozen were stolen during the riot); and the agency’s First Responder Unit was not equipped with adequate less-lethal weapons, among other findings.

His previous reports have found that the Capitol Police had clearer warnings about the Jan. 6 attack than were previously known, including the potential for violence in which “Congress itself is the target.” But officers were instructed by their leaders not to use their most aggressive tactics to hold off the mob.

About 140 officers were injured during the attack, and seven people died in connection with the siege, including one officer who suffered multiple strokes after sparring with members of the mob.

“It is our duty to honor those officers who have given their lives but also ensuring the safety of all those working and visiting the Capitol Complex by making hard changes within the department,” Mr. Bolton said in written testimony.

At a previous hearings on the attack, some House Republicans have used the venue to attempt to rewrite the history of what happened on Jan. 6, downplaying or outright denying the violence and deflecting efforts to investigate it.

Republicans signaled Monday that they planned to focus on Chief Pittman’s lack of attendance.

In response, the Capitol Police cited Chief Pittman’s need to hear from the inspector general, and said she would testify before the committee another time when there wasn’t a conflict. She has testified in previous committee hearings reviewing the attack, including when she apologized on behalf of the department.