WASHINGTON (AP) — Troops in riot gear lined the sidewalks, but there were no crowds. Armored vehicles and concrete barriers blocked empty streets. Miles of fencing cordoned off many of the nation's most familiar landmarks.

Joe Biden was safely sworn in as president in a Washington on edge, two weeks after rioters loyal to former President Donald Trump besieged the Capitol.

Law enforcement officials contended not only with the potential for outside threats but also with rising concerns about an insider attack. Officials monitored members of far-right extremist and militia groups, increasingly concerned about the risk they could stream into Washington and spark violent confrontations, a law enforcement official said.

There were a few scattered arrests but no major protests or serious disruptions in the city during Biden's inauguration ceremony.

As Biden put it in his address: “Here we stand just days after a riotous mob thought they could use violence to silence the will of the people, to stop the work of our democracy, to drive us from this sacred ground. It did not happen. It will never happen, not today, not tomorrow, not ever. Not ever.”

After the deadly attack that killed five on Jan. 6, the Secret Service stepped up security for the inauguration early, essentially locking down the nation's capital. More than 25,000 troops and police were called to duty. The National Mall was closed. Checkpoints were set up at intersections.

In the hours before the event, federal agents monitored “concerning online chatter,” which included an array of threats against elected officials and discussions about ways to infiltrate the inauguration, the official said.

In right-wing online chat groups, believers in the QAnon conspiracy theory expressed disappointment that top Democrats were not arrested for sex trafficking and that Trump did not seize a second term.

Twelve National Guard members were removed from the security operation a day earlier after vetting by the FBI, including two who had made extremist statements in posts or texts about Wednesday's event. Pentagon officials would not give details on the statements. The FBI vetted all 25,000 members in an extraordinary security effort in part over the presence of some ex-military in the riot.

Two other U.S. officials told The Associated Press that all 12 were found to have ties with right-wing militia groups or to have posted extremist views online. The officials, a senior intelligence official and an Army official briefed on the matter, did not say which fringe groups the Guard members belonged to or what unit they served in. The officials told the AP they had all been removed because of “security liabilities.”

The officials were not authorized to speak publicly and spoke on condition of anonymity.

Gen. Daniel Hokanson, chief of the National Guard Bureau, confirmed that Guard members had been removed and sent home, but said only two cases were related to inappropriate comments or texts related to the inauguration. He said the other 10 cases were for issues that may involve previous criminal behavior or activities but were not directly related to the inaugural event.

The FBI also warned law enforcement officials about the possibility that members of right-wing fringe groups could pose as National Guard troops, according to two law enforcement officials familiar with the matter.

Investigators in Washington were particularly worried that members of right-wing extremist groups and militias, like the Oath Keepers and Three Percenters, would descend on Washington to spark violence, the law enforcement officials said. Some of the groups are known to recruit former military personnel, to train extensively and to have frequented anti-government and political protests.

In addition to the thousands of National Guard troops, hundreds of law enforcement officers from agencies around the country were also brought into Washington. The increased security is likely to remain in the nation's capital for at least a few more days.

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Associated Press writers Lolita Baldor in Washington and James LaPorta in Delray Beach, Florida, contributed to this report.