WASHINGTON — As House leaders haggle over the formation of a 9/11-style commission to investigate the Jan. 6 attack on the Capitol, a bipartisan group of senators is pressing ahead with a series of investigative hearings to scrutinize the security breakdowns that failed to prevent the deadly pro-Trump rampage.
The inquiry begins on Tuesday with a joint hearing of two Senate committees to question the officials who were in charge of securing the Capitol during the attack, when Capitol Police officers and members of the District of Columbia’s police force called in as reinforcements were overrun as the vice president and members of the House and the Senate were gathered inside.
The meeting of the Homeland Security and Governmental Affairs Committee and the Rules and Administration Committee will be the first time the public has heard from the top two security officials at the Capitol on the day of the assault, both of whom resigned after the breach.
Paul D. Irving, the former House sergeant-at-arms, and Michael C. Stenger, the former Senate sergeant-at-arms, have come under scrutiny amid reports that they did not act swiftly enough to call for the National Guard. The committees will also hear from Steven A. Sund, the former chief of the Capitol Police, who has also resigned, and Robert J. Contee, the chief of the Metropolitan Police Department.
“I support the commission,” said Senator Amy Klobuchar, Democrat of Minnesota and the chairwoman of the administration panel, which oversees Capitol security. “But it’s important to get the information out under oath as soon as possible. While the 9/11-type commission may be going on, decisions have to be made about the Capitol, sooner rather than later.”
“They are all attending voluntarily,” Ms. Klobuchar said of the witnesses.
The attack left nearly 140 police officers injured and several people dead. Mr. Sund previously said in a letter to Congress that the sergeants-at-arms turned down his request for the National Guard ahead of Jan. 6 and did not respond quickly that day when he urgently called for troops to help his officers.
The hearing will be the first in a series of oversight hearings organized by Ms. Klobuchar and Senator Gary Peters, Democrat of Michigan and the chairman of the Homeland Security and Governmental Affairs Committee, along with the top Republicans on both panels, Senators Roy Blunt of Missouri and Rob Portman of Ohio.
But already, the move to investigate the Capitol riot — the deadliest attack on the building where Congress convenes in 200 years — has turned political. Republicans are resisting a proposal by Speaker Nancy Pelosi, Democrat of California, to form an independent, bipartisan commission modeled after the one that investigated the Sept. 11 terrorist attacks, arguing that her blueprint would skew the commission toward Democrats.
Under Ms. Pelosi’s outline, according to two Democrats familiar with it, each of the top four congressional leaders would nominate two members and President Biden would name three, including the commission chair.
“It is our responsibility to understand the security and intelligence breakdowns that led to the riots on Jan. 6 so that we can better protect this institution and the men and women working inside it,” Representative Kevin McCarthy of California, the minority leader, said in a statement invoking the chairmen of the 9/11 Commission. “A commission should follow the guidance of Tom Kean and Lee Hamilton to be ‘both independent and bipartisan,’ and to preserve that integrity it must be evenly split between both parties.”
The 10-member National Commission on Terrorist Attacks Upon the United States, as the Sept. 11 panel was formally known, was the product of an intense round of negotiating on Capitol Hill and was evenly split between Republicans and Democrats.
In the Senate, the next investigative hearing, whose date has not yet been scheduled, will focus on the F.B.I., the Department of Homeland Security, the Defense Department and the threat of domestic extremism, according to the senators and their aides.
“Decisions have to be made about how we can improve the coordination of security information,” Ms. Klobuchar said. “Decisions have to be made about the new people who will take these jobs. You have to see the whole landscape here about what went wrong and how we can do better.”
Mr. Peters said he believed there would be multiple inquiries into the attack on the Capitol. During the hearing on Tuesday, Mr. Peters said, he would question the security officials on intelligence failures ahead of Jan. 6, communication breakdowns and a slow response to call for help.
“This will be the first time these officials will be speaking publicly,” Mr. Peters said. “By all appearances, there was not adequate preparation. Clearly, there was a failure of leadership.”
Mr. Peters said he also expected lawmakers to question why the police presence was weaker for the “Save America Rally” on Jan. 6 than it was for Black Lives Matter protests during the summer. Mr. Peters led a bipartisan group of senators in sending a letter to 22 agencies requesting detailed explanations of their security preparation and response that day.
In a statement, Mr. Portman said that both Republicans and Democrats wanted to get to the bottom of the security failures at the Capitol that day.
“I look forward to hearing directly from the officials responsible for securing the Capitol complex who made the decisions regarding preparations and response efforts that led to the security failures that endangered the lives of Vice President Pence, members of Congress, first responders and staff,” Mr. Portman said. “This will inform what reforms need to be made to ensure nothing like Jan. 6 ever happens again.”