Congress will be hearing from the former U.S. On Jan. 6, the day a violent mob laid siege to the building and disrupted the presidential electoral count, Capitol security officials first addressed the major law enforcement failures.

Three out of the four set to testify Tuesday before two Senate panels, including the former head of the Capitol Police, resigned under pressure shortly after the deadly assault.

Much remains unclear about what happened before and after the attack, and it is expected that lawmakers would question the former officials vigorously about what went wrong. How much did law enforcement authorities, many of whom were public, know about preparations for violence that day? How have the agencies exchanged their data with each other? And how could the Capitol Police have been so ill-prepared for a violent rebellion that, in plain sight, was orchestrated online?

On the outside of the Capitol, the rioters quickly smashed through security barriers, engaged in hand-to-hand battles with police officers, wounding hundreds of them, and crashed through several windows and doors, sending lawmakers running from the chambers of the House and Senate, and interrupting the 2020 presidential election certification. As a result of the shooting, five people died, including a Capitol Police officer and a woman shot by police as she tried to break through the House chamber doors with lawmakers still inside.

Former Senate Sergeant-at-Arms Michael Stenger and former House Sergeant-at-Arms Paul Irving will talk publicly at the hearing, which is part of a joint inquiry by the Senate Committee on Homeland Security and Government Affairs and the Senate Rules Committee, for the first time since their resignations. They will be accompanied by former Capitol Police Chief Steven Sund and Robert Contee, the Metropolitan Police Department’s acting chief of police, who, after the rioting began, sent additional officers to the scene.

The hearing is supposed to be the first of many analyses of what happened that day, occurring almost seven weeks after the assault and more than a week after the Senate voted to acquit former President Donald Trump of triggering the rebellion by urging his supporters to “fight like hell” to undo his election defeat. The Capitol is now surrounded by thousands of National Guard troops on a large perimeter, cutting off streets and sidewalks that are typically full of vehicles, pedestrians and visitors.

A bipartisan, impartial commission to investigate the mistakes is also being considered by Congress, and numerous Congressional panels have said they would look into various aspects of the siege. More than 230 individuals who were suspected of being involved in the attack were arrested by federal law enforcement, and President Joe Biden’s nominee for attorney general, Judge Merrick Garland, said in his confirmation hearing Monday that investigating the riots would be a top priority.

In an interview with The Associated Press, Congress needs to know, quickly, how failed security plans and delays in the response led to “a mad, angry mob invading this temple of our democracy,” Senate Rules Committee Chairwoman Amy Klobuchar said.

Klobuchar, D-Minn., said senators would concentrate in particular on the timing of the deployment of the National Guard, which finally arrived to assist the exhausted police, how security agencies exchanged information prior to the attack, and if the Capitol Police Board’s command structure, which includes the armed sergeants of the House and Senate, led to the failures. She said laws could exist to resolve any inadequacies.

Here we are on a fast track simply because decisions about the Capitol have to be made,” Klobuchar said.”

Klobuchar said the hearing on Tuesday would be the first of at least two public reviews of what went wrong that day, as the Senate committees are conducting a joint security failure inquiry. The reaction of the Defense Department, the Department of Homeland Security and the FBI will be investigated at a second hearing, scheduled to take place in the next few weeks.

Although there is general consensus that security measures on that day were ineffective, officials have pointed to each other’s responsibility for the causes and questioned the accounts of each other. It soon became apparent that although the Capitol Police had prepared for demonstrations, they were greatly unprepared for a violent rebellion, and many were battered as they tried in vain to prevent rioters from entering the building. The day after the riot, Sund said that his force “had a robust plan established to address expected First Amendment activities.”

Interim Capitol Police Chief Yogananda Pittman, who briefly replaced Sund, apologised last month for failing to plan, amid threats that Congress would be threatened by white nationalists and far-right groups. She also reported, however, that Sund had demanded that the Capitol Police Board, which oversees the department, declare a state of emergency in advance and allow him to request assistance from the National Guard, but the board refused. The Department of Defense has said it asked the Capitol Police if the Guard wanted it, but the request was refused.

Pittman’s assertion was refuted by a third member of the Capitol Police Board hours after her testimony was released. J. Brett Blanton, who serves as the Capitol’s architect, said Sund had not asked him for support and that there was “no record of an emergency declaration request.”

By interrogating the witnesses together on Tuesday, lawmakers expect to settle some of those contradictions. Klobuchar said she is glad that they are all willingly attending and hopes that a “constructive” tone will be offered to the hearing.

“What happened was a horror, all of us know that,” she said. “But if we’re going to have solutions and move forward with a safer Capitol, we need to identify what went wrong, what the problems were, and the answers we’re going to get are part of that solution.”