Moving the ceremony indoors to the Capitol because of the freezing weather on Monday may lessen some risks but major threats remain.

"From radiological threats to flat tires, the Secret Service has spent over a year preparing for Monday's inauguration, working with top law enforcement and public safety partners to ensure a safe, successful event," Secret Service Spokesperson Anthony Guglielmi posted on X.

More than 25,000 personnel from around the country will be deployed for security, Secret Service Special Agent Matt McCool said at a news conference on Sunday.

They will include National Guard contingents from 40 states and police volunteered by 32 states to supplement the national capital's police forces.

Helicopters and drones will provide security from the air and miles of fences will gird the Capitol and key areas.

Space agency NASA said it was conducting low-altitude helicopter flights over Washington to check for radiation risks.

Giving reassurances that there won't be a repeat of the assassination attempts against Trump, McCool said, "We took a close look at what happened there, and those things that went wrong have been fixed. You will not see something like that happen here at the inauguration."

The New Year's Day attack using a vehicle by an Islamic State terrorist on celebrators in New Orleans has brought another threat to the mix.

McCool said that there will be two layers of fencing around the Capitol and reinforced with vehicles capable of blocking attacks and concrete barriers.

Although the ceremony will be held at the rotunda of the Capitol, which houses the two chambers of Congress, with very limited capacity, and a livecast will be shown at the Capital Arena for about 20,000 people, thousands will brave the freezing temperatures and gather outdoors.

It could also draw protesters like those who participated in what was billed as the "People's March" on Saturday.

Politico reported that several intelligence and police agencies have warned that the inauguration could be an "attractive target" for extremists.

In a secret threat assessment document, the group of agencies warned that "foreign terrorists, domestic extremists, and lone wolves could initiate bomb hoaxes, swatting calls (fake complaints to divert security), drone flights or vehicle-ramming attacks," according to Politico.