A dozen members of the United States National Guard were removed from duty in Washington DC ahead of President-elect Joe Biden's inauguration, The Associated Press reported on Wednesday.

The members were removed from the security operation after vetting by the Federal Bureau of Investigation (FBI) and included two who had made extremist statements in posts or texts Wednesday's event.

Pentagon officials wouldn’t give details on the statements.

Two other US officials told AP 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 authorised 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 potential issues that may involve previous criminal behaviour or activities but were not directly related to the inaugural event.

Their removal from the massive security presence at the nation’s capital came amid worries from US defence officials about a potential insider attack or other threat from service members following the deadly riot at the US Capitol on January 6 by Trump supporters. The FBI has been working to vet all 25,000 National Guard in town.

Officials have said the Pentagon has found no intelligence so far that would indicate an insider threat. But law enforcement officials are contending not only with the potential for outside threats but also with rising concerns about an insider attack by troops with a duty to protect the president-elect.

There have been no specific threats made against Biden.

The nation’s capital is essentially on lockdown. More than 25,000 troops and police have been called to duty. Tanks and concrete barriers block the streets. The National Mall is closed. Fencing lines the perimeter of the US Capitol complex. Checkpoints sit at intersections. The US Secret Service, which is in charge of the event, says it is prepared.

But law enforcement officials have been monitoring members of far-right extremist and militia groups. They have grown increasingly concerned about the possibility such groups could stream into Washington and spark violent confrontations, a law enforcement official said.

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

The FBI has also warned law enforcement officials about the possibility that right-wing fringe groups could pose as members of the National Guard, according to two law enforcement officials familiar with the matter. Over the summer, a man carrying a handgun and an assault rifle was arrested in Los Angeles and charged with impersonating a National Guard member during a protest. Actual Guardsmen confronted him when they noticed things out of place on his uniform.

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

That concern intensified significantly after investigators identified members of right-wing extremist groups participating in the Capitol riot.

Washington has been on edge since the deadly insurrection. A fire in a homeless camp roughly a mile from the Capitol complex prompted an evacuation on Monday during a rehearsal for the inauguration. The arrests of two people with guns who entered the checkpoints set off concerns, though the arrests had no apparent connection to the inauguration.

Federal law enforcement officials have also been wary of increased surveillance of military and law enforcement checkpoints and other positions. Some National Guard troops have reported people taking pictures and recording them, said the law enforcement officials, who spoke to the AP on condition of anonymity to discuss ongoing security matters.

In a related problem, the Secret Service issued a bulletin over the weekend about what it sees as an “uptick” in National Guard troops posting pictures and details of their own operations online.

The AP obtained the message sent to all National Guard troops coming to Washington. The bulletin read: “No service members should be posting locations, pictures or descriptions online regarding current operations or the sensitive sites they are protecting” and urged them to stop immediately.

Asked about the bulletin, a spokesperson for the Secret Service said the agency “does not comment on matters of protective intelligence".

Neither Hokanson nor Pentagon spokesperson Jonathan Hoffman would provide details on the comments or texts made by the two Guard members. Speaking at a Pentagon news conference, Hokanson said one was identified by his chain of command and the other was identified through an anonymous tip.

“Much of the information,” Hoffman said, “is unrelated to the events taking place at the Capitol or to the concerns that many people have noted on extremism. These are vetting efforts that identify any questionable behaviour in the past or any potential link to questionable behaviour, not just related to extremism.”

Hoffman said officials aren’t asking questions right now of those who were flagged.

But later, he said, “we will address them, whether it’s through law enforcement, if necessary, or through their own chain of command."