“We’ve done this four years ago and even four years before then. But this is the first time we’re putting in place a plan where there’s been an active assault on the institutions of our democracy,” Dotson said he told his officers. “This is as serious of a law enforcement threat as I’ve seen, potentially, in my career.”

Dotson said Amtrak's Union Station in Washington, DC, was relatively calm during the January 6 riots, and in fact people seeking to escape the unrest ended up sheltering there. The cavernous railway station and shopping area is just a few blocks from the Capitol complex.

“When the activities at the Capitol turned violent, the people that ... didn’t want anything to do with that sought refuge,” Dotson said. “So they came to our station.” Still, he said his officers had to confront some rally-goers who didn't like Amtrak’s mask requirement.

How to protect members of Congress while in airports is of particular concern, after several viral videos last week showed lawmakers trying to catch flights being encircled by aggressive crowds. Senator Lindsey Graham, one of President Donald Trump’s most loyal allies on Capitol Hill, was not loyal enough for throngs of Trump supporters who surrounded him at Ronald Reagan Washington National Airport, yelling “traitor”.

Hoping to head that off next week, airports, airlines and security agencies have begun intense collaborations, including keeping tabs on which flights will be carrying lawmakers. Plans are also being made to ensure lawmakers will have safe places to wait, such as airline lounges, and potentially escorts in public areas, according to an airport representative familiar with the discussions. In some cases, federal marshals may accompany their flights.

Meanwhile, FAA has warned that it will throw the book at unruly passengers following multiple incidents of people disrupting flights in and out of Washington.

“Over the last few days, we have seen a disturbing increase in on-board incidents where airline passengers have disrupted flights,” FAA Administrator Steve Dickson told CNBC Thursday. “These incidents have stemmed, in some cases, from refusal to follow airline policies on face coverings, and also we saw a trend after the breach of the Capitol last week.”

FAA has the authority to fine passengers up to US$35,000 and seek prison time if their behaviours threaten other passengers or the aircraft. The agency said it will be turning to those tougher penalties instead of warnings and other slaps on the wrist that it has pursued in some past cases. FAA said the new policy will be in effect until March 30.

Calls have grown for Capitol rioters to be added to the federal no-fly list, including from incoming Senate Majority Leader Chuck Schumer and members of the House Homeland Security Committee.

The FBI has not disclosed whether it has added anyone who took part in the January 6 riots to the list, but said in a statement to Politico that it “will continue to nominate predicated subjects to the federal terrorism watch list, as appropriate, in accordance with existing laws and policies.”

The agency further noted that agencies and local law enforcement could detain or arrest any individual considered to pose an immediate threat to other airline passengers or the aircraft, “which would effectively prevent them from flying.” And airlines, which maintain their own lists of people who are no longer welcome on their planes, could deny people service if they are disruptive.

In a statement, TSA Administrator David Pekoske said his agency is working to ensure that anyone who may pose a threat undergoes “enhanced screening” or is prevented from getting on board an aeroplane. In addition, he said extra security has been put into place at the area's three airports, including canine teams and increased numbers of air marshals on flights. TSA agents will also bolster other security forces deployed around the city, including assisting with screening people along the parade route and at the inauguration itself.

03:15

World shocked by assault on the US Capitol by radical pro-Trump supporters in Washington

World shocked by assault on the US Capitol by radical pro-Trump supporters in Washington

Washington, DC's transit system is preparing, too. Last week saw Metrorail station managers and cleaners advised by their union – Amalgamated Transit Union Local 689 – to lock themselves in back rooms to protect themselves.

“A lot of those folks who came through here refused to wear masks,” said ATU Local 689 Vice-President and Chief Safety Officer Carroll Thomas. “They had weapons. They had all kinds of things. Our members were afraid for their lives.”

Metro has announced that it will close 13 stations within the security perimeter around the White House and Capitol grounds starting on Friday through the Thursday after the inauguration.

In addition, Amtrak and its employee unions have petitioned the federal government to expand TSA’s no-fly list to passenger rail.

The groups said they want a “proportional presence of workers in the passenger compartments of Amtrak trains similar to that in the airline industry, with at least one conductor or assistant conductor present per 50 riders,” according to a statement issued this week.

Though most concerns so far are centring on the inauguration and the days immediately around it, the increased vigilance and security put in place for this event may not end quickly.

“I’m very worried about next week as the inauguration week but I don’t think the week after all of a sudden people are going to be like, ‘Let’s all move on,’ and hunky dory,” said the airport official. “I think we’re in for some period of time of increased tensions in our country, and that will impact airports as well as pretty much every other part of society.”