The Justice Department is forming a new domestic terrorism unit to help combat a threat that has intensified dramatically in recent years, a top national security official said Tuesday.

Matthew G. Olsen, the head of the Justice Department’s national security division, announced the creation of the unit in his opening remarks before the Senate Judiciary Committee, noting that the number of FBI investigations of suspected domestic violent extremists — those accused of planning or committing crimes in the name of domestic political goals — had more than doubled since the spring of 2020.

Olsen said that the Justice Department previously had counterterrorism attorneys who worked both domestic and international cases and that the new unit would “augment our existing approach.”

His testimony came just a few days after the anniversary of the Jan. 6, 2021, riot at the Capitol, an event that some lawmakers say showed that the FBI underestimated the threat posed by domestic extremists and violence-prone members of far-right groups.

“This group of dedicated attorneys will focus on the domestic terrorism threat, helping to ensure that these cases are handled properly and effectively coordinated across the Department of Justice and across the country,” Olsen said.

Domestic terrorism data shows extremist violence is on the rise in America. Here’s how lawmakers and the FBI are responding. (Sarah Hashemi, Monica Rodman, Monica Akhtar/The Washington Post)

The hearing was convened to assess the threat of domestic terrorism a year after the Jan. 6 attack. It often devolved into partisan bickering over the riot, which involved hundreds of Trump supporters who marched to the Capitol after a rally outside the White House, and the violence and looting that erupted at some racial justice protests in 2020.

Sen. Richard J. Durbin (D-Ill.) opened the hearing with a video showing footage and news coverage from the Jan. 6 riot, taking aim at Republicans for not being fully supportive of congressional efforts to investigate the attacks on police officers, threats against lawmakers and attempts to undo Joe Biden’s electoral victory.

“They are normalizing the use of violence to achieve political goals,” Durbin said.

Sen. Charles E. Grassley (R-Iowa) countered with a video showing unrest in Portland, Ore., and elsewhere. “These anti-police riots rocked our nation for seven full months, just like the January 6 assault on the Capitol rocked the nation,” he said.

Sen. Ted Cruz (R-Tex.) lambasted Olsen and Jill Sanborn, the head of the FBI’s national security branch, for not answering certain questions about Jan. 6-related criminal charges or about whether any FBI informants encouraged or participated in the violence.

“Your answer to every damn question is ‘I don’t know, I don’t know, I don’t know,’ ” Cruz railed at Olsen. To Sanborn, he suggested that undercover FBI agents or informants may have spurred on the rioters — an assertion for which there is no known evidence but which Sanborn would not categorically rule out.

“Ms. Sanborn, a lot of Americans are concerned that the federal government deliberately encouraged illegal violent conduct on January 6th,” Cruz said, asking her whether that was true.

“Not to my knowledge, sir,” she replied.

She and Olsen sought to assure lawmakers that the Justice Department is investigating and prosecuting all of those who committed crimes, no matter what motivated them. Olsen said authorities had arrested and charged more than 725 people, including more than 325 facing felony counts, in connection with their roles in the Jan. 6 attack.

The FBI is seeking to identify and arrest more than 200 additional suspects.

Sanborn said the bureau had opened more than 800 cases in connection with the riots during the summer 2020 and arrested more than 250 people.

The bureau assessed racially or ethnically motivated violent extremism and anti-government violent extremism as being the most “lethal” terrorism threats, Sanborn said. She added that the FBI had recently elevated anti-government violent extremism as a priority, on par with racially motivated violent extremism, homegrown violent extremism and extremism planned or inspired by the Islamic State.

The breach of the Capitol has spurred new political and policy debates about failures of the FBI and other law enforcement agencies to prevent the attack, and about how the government combats domestic terrorism.

The Justice Department and the bureau have faced criticism in recent years for not focusing as intensely on domestic terrorism as they do internationally inspired threats, though officials have insisted they take both matters seriously.

Last year, the White House released a national strategy to address the problem, calling for, among other things, new spending at the Justice Department and FBI to hire analysts, investigators and prosecutors.

Historically, domestic terrorism investigations come with more procedural and legal hurdles than cases involving suspects inspired by groups based outside the United States, such as the Islamic State or al-Qaeda. The charge of material support for a foreign terrorist group, for instance, has no legal equivalent for someone eager to commit violence in the name of domestic political goals.

As a result, domestic terrorism investigators frequently settle for filing gun or drug charges, and often those are filed in state — not federal — court, which can mask the overall extent of extremist threats.

Democrats pressed Olsen to explain why prosecutors had not sought tougher sentences in Jan. 6 cases by seeking enhancements for terrorism. Olsen did not answer the question directly, saying each case had to be evaluated on its particular facts. He pointed to recent remarks from Attorney General Merrick Garland, who suggested such enhancements could come as prosecutors win convictions in more-serious cases.

Where precisely to draw lines about who is or who isn’t a domestic terrorist is also a subject of debate. At the hearing, Sanborn said that last year, there were four attacks conducted by domestic violent extremists, resulting in 13 deaths. She did not identify or describe the incidents, and FBI officials did not provide any details in response to questions from The Washington Post in the hours after the hearing.

Federal law defines domestic terrorism as criminal acts within the United States that are dangerous to human life and that appear to be intended “to intimidate or coerce a civilian population . . . to influence the policy of a government by intimidation or coercion; or . . . to affect the conduct of a government by mass destruction, assassination, or kidnapping.”

After the Jan. 6 attack, Democrats said the Justice Department and FBI did not aggressively pursue domestic terrorism cases during the Trump administration.

From 2016 to 2019, the number of domestic terrorism suspects arrested per year fell from 229 to 107, before jumping to 180 in 2020. Since 2020, the number of open investigations has grown rapidly. FBI Director Christopher A. Wray has previously said that to handle the caseload, he more than tripled the number of agents and analysts working on domestic terrorism cases.

At Tuesday’s hearing, conservative lawmakers pushed Olsen and Sanborn to explain and justify a memo Garland wrote last year urging more scrutiny of threats against school officials. Grassley said the memo had “a chilling effect on freedom of speech.”

Olsen and Sanborn said the school-threats issue was a small part of their divisions’ work. Olsen said such matters were largely handled by other Justice Department components — including the criminal division and civil rights division — while the national security division played an “advisory role.”

Sanborn said the issue “is not a particular focus for the counterterrorism division,” adding that the FBI would only get involved if there was an allegation of a violation of federal law. She also sought to downplay the significance of the FBI’s efforts to track such cases by applying a particular record-keeping tag, saying that was “simply an administrative process to be able to better analyze trends.”