The commanding general of the D.C. National Guard will testify on Wednesday during a Senate hearing about the attack on the U.S. Capitol, joining a group that initially included only civilian witnesses, defense and congressional officials said.

The investigation — which congressional officials said could take more than a year — is far-reaching, with requests for information from multiple agencies and departments. A witness list disseminated to the news media last week included senior civilian officials from the Defense Department, FBI and Department of Homeland Security, but Maj. Gen. William Walker, the commanding general of the D.C. National Guard, was added to the list on Monday.

Walker oversaw the D.C. National Guard in its limited role responding to the possibility of violence ahead of the riot at the Capitol on Jan. 6, which was spawned from a rally in support of President Donald Trump’s false claims that the election was stolen from him. The National Guard response was expanded hours after Congress was overrun, eventually including about 26,000 National Guard members through the Jan. 20 inauguration of President Biden. Several thousand remain activated in Washington to provide security.

Capt. Edwin Nieves, a D.C. National Guard spokesman, said Monday that Walker is open to testifying and cooperating with the investigation.

The initial lineup of witnesses did not include any senior military officers, effectively shielding the military’s brass from scrutiny during a two-part joint hearing by the Senate Homeland Security and Governmental Affairs Committee and Senate Committee on Rules and Administration that began last week. In the first part, police chiefs and senior congressional security officials repeated their frustrations about what they described as a slow Defense Department response as Congress was violently breached.

Many have argued that President Donald Trump's efforts amounted to an attempted coup on Jan. 6. Was it? And why does that matter? (Monica Rodman, Sarah Hashemi/The Washington Post)

“I was very surprised at the amount of time and the pushback I was receiving when I was making an urgent request for their assistance,” said former Capitol Police chief Steven Sund, who resigned in the aftermath of the attack.

Acting D.C. police chief Robert J. Contee III went further, saying he was “stunned” by the response from the Department of the Army, calling senior Army officials “reluctant” to send the D.C. National Guard to the Capitol.

“While I certainly understand the importance of both planning and public perception — the factors cited by the staff on the call — these issues become secondary when you are watching your employees, vastly outnumbered by a mob, being physically assaulted,” Contee testified.

Defense officials have previously said they scrambled to get National Guard members to respond to the violence but were hamstrung by a lack of a plan by Capitol Police on how they might assist, and agreements with D.C. officials that were designed to limit National Guard involvement in the city. Those discussions came after thousands of National Guard members were deployed by the Trump administration in Washington last summer in response to protests for racial justice and unrest.

Committee members initially invited Defense Secretary Lloyd Austin or his designee to participate in Wednesday’s hearing for the Defense Department, according to three officials familiar with the situation, who spoke on the condition of anonymity because of the sensitivity of the issue. Austin tapped Robert Salesses, who is performing the duties of an assistant secretary of defense, to testify, they said.

Other witnesses expected to testify include Jill Sanborn, the assistant director of the FBI’s counterterrorism divisions; and Melissa Smislova, a senior official performing the duties of an undersecretary in the Department of Homeland Security’s office of intelligence and analysis.

On Friday, two Homeland Security and Governmental Affairs Committee officials defended the decision not to have a military officer testify this week.

“Our goal is to emphasize the civilian control of the military and the decision-makers on that day, and so having the deputy assistant secretary that covers all of the Pentagon support for civilian authorities is going to give us the best shot at getting those answers,” one of the officials said.

But on Monday, that changed.

Walker, as the D.C. National Guard’s top officer, reports to senior civilian leaders in the Army. He has said in interviews that in the days leading up to the pro-Trump rally, he received an informal request from Sund to be ready in case violence broke out.

But Walker said he never received a formal request to prepare for a crisis from anyone.

“We have people in this armory every day,” he told The Washington Post on Jan. 16. “But we never got an official request that has to go up the chain of command. We didn’t get that until the day of, and the Capitol already was under duress.”

Walker said in another interview with The Post on Jan. 26 that senior Pentagon officials had restricted his ability to respond ahead of the riot, requiring higher-level sign-off that cost time as violence erupted.

“All military commanders normally have immediate response authority to protect property, life, and in my case, federal functions — federal property and life,” Walker said. “But in this instance I did not have that authority.”

Governors serve as the de facto commanders in chief of National Guard forces in their states. But in the District, which does not have statehood, the president has control of the National Guard and typically delegates it to the defense secretary and Army secretary.

During the riot, then-Army Secretary Ryan McCarthy had control of the D.C. National Guard. He has not been called to testify but has spoken with Senate officials about the response to the attack on several occasions, said committee officials and a defense official familiar with the issue.

The Senate has the ability to subpoena anyone it sees as necessary to the investigation. One committee official said that power will be used only as a last resort.