WASHINGTON — The F.B.I. director warned senators on Tuesday that domestic terrorism was “metastasizing across the country,” reaffirming the threat from racially motivated extremists while largely escaping any tough questions about the bureau’s actions before the siege of the Capitol.

The director, Christopher A. Wray, who had largely remained out of public view since the riot on Jan. 6, condemned the supporters of former President Donald J. Trump who ransacked the Capitol, resulting in five deaths and scores of injuries to police officers.

“That attack, that siege, was criminal behavior, plain and simple, and it was behavior that we, the F.B.I., view as domestic terrorism,” Mr. Wray said. “It’s got no place in our democracy.”

He also revealed that the number of domestic terrorism investigations at the F.B.I. had risen to 2,000 since he became its director in 2017. The Capitol riot was part of a broader threat that had grown significantly in recent years, Mr. Wray said.

He did not break down the inquiries along an ideological divide, but The New York Times has reported that agents opened more than 400 domestic terrorism investigations last year as violence flared during racial justice protests, including about 40 cases into possible adherents of the far-left antifascist movement known as antifa and another 40 into the Boogaloo, a far-right movement seeking to start a civil war. The F.B.I. also investigated white supremacists suspected of menacing protesters.

Mr. Wray’s appearance before the Senate Judiciary Committee was his first in front of Congress since the assault on the Capitol. It was free of the drama after similar testimony last year, when Mr. Trump — who appointed Mr. Wray to his post — attacked him for detailing the threat from far-right extremists and stoked a false narrative that anti-fascists were the real danger. In contrast, the Biden administration has made fighting domestic terrorism a priority.

As a result of the violence last year, the F.B.I. and the Justice Department decided to elevate the threat posed by antigovernment and anti-authority extremists such as militias and anarchists. Still, bureau officials listed the threat a tier below the one presented by racially motivated violent extremists like neo-Nazis.

The F.B.I. and the Justice Department make those determinations based on violent attacks such as shootings or bombings and use the levels to decide where to focus resources.

Mr. Wray pointed out another alarming trend: The number of white supremacists arrested in 2020 had almost tripled from when he started running the F.B.I. three years earlier.

White supremacists have killed dozens of people in the United States since 2015, opening fire at a Black church in South Carolina and at synagogues in Pittsburgh and California, as well as targeting Hispanic shoppers at a Walmart in Texas.

The political implications of the threats played out at the hearing. While Republicans condemned the Capitol attack, some were quick to point to unrest last year in Portland, Ore., and other cities, highlighting the destruction of property and attacks on the police. In one spasm of violence, a self-professed antifa supporter shot to death a pro-Trump protester in Portland in August.

Still, it was the first killing in more than 20 years by what the bureau classifies as an “anarchist violent extremist.”

Mr. Wray repeatedly said in response to questions from Democratic senators that people associated with antifa were not involved in storming the Capitol and that rioters were genuinely Trump supporters, not posing falsely as them.

Senator Richard J. Durbin of Illinois, the Democratic chairman of the committee, accused the Trump administration of playing down the threat from white supremacists while stoking a narrative that left-wing anarchists such as those who identify with antifa were the greater danger to the country.

Rattling off the litany of mass shootings, Mr. Durbin added, “Let’s stop pretending that the threat of antifa is equal to the white supremacist threat.”

The Capitol Police has largely shouldered the blame for the Jan. 6 attack. Its acting chief, Yogananda D. Pittman, has acknowledged to Congress that the authorities failed to do enough to thwart the “terrorist attack.”

Indeed, there were several indicators of the potential for violence on Jan. 6. Federal law enforcement officials knew that members of militias such as the Oath Keepers and far-right groups such as the Proud Boys planned to travel to Washington, some potentially with weapons. Many adherents of QAnon, a dangerous conspiracy theory that has emerged as a possible domestic terrorism threat, were also expected to attend a protest rally where Mr. Trump spoke before the attack.

In addition, the F.B.I.’s office in Norfolk, Va., produced a report a day earlier warning of possible violence and mentioned people sharing a map of tunnels at the Capitol complex. However, the information was unverified, and a portion quoting a warning of an impending “war” appeared to come from a single online thread.

The F.B.I. provided the report to the Capitol Police, although its former chief, Steven A. Sund, has said it never made it up the ranks.

Mr. Wray said that F.B.I. officials relayed the Norfolk information on at least three occasions to other law enforcement agencies. He said that he had not seen the report until after the riot, but that the handling of it was typical for such intelligence.

Senator Lindsey Graham, Republican of South Carolina, asked what Capitol Police leaders should have done had they seen the Jan. 5 report.

“I really want to be careful not to be an armchair quarterback,” Mr. Wray said. He later said he did not have a “good answer” as to why Mr. Sund did not get the report.

With the signs pointing to violence or worse on Jan. 6, Senator Richard Blumenthal, Democrat of Connecticut, pressed Mr. Wray on why the F.B.I. did not “sound the alarm in some more visible and ringing way.”

Mr. Wray said the bureau had for months released intelligence reports related to domestic terrorism — some specifically tied to the election — publicly and to other law enforcement agencies such as the Capitol Police.

He said the bureau was reviewing its actions but agreed that the insurrection was not an “acceptable result.”

“We aim to bat a thousand,” Mr. Wray said.

But it was clear that federal law enforcement underestimated the potential for violence on Jan. 6 among Trump supporters, many of whom portrayed themselves as backers of law enforcement.

The focus on antifa among Mr. Trump and some of his cabinet officials and the shifting of law enforcement sources last spring and summer might have contributed to the F.B.I. failing to heed the rising anger among Mr. Trump’s supporters about false claims of election fraud that culminated in the storming of the Capitol, current and former law enforcement officials have said. Mr. Trump himself had pushed that conspiracy theory, influencing his followers with the baseless notion that the election had been stolen.