US airlines and law enforcement agencies have bolstered security at Washington-area airports on Thursday after supporters of Donald Trump caused mayhem in the US capital in an attempt to overturn his election loss.

“Passengers can expect to see an increased law enforcement presence as they travel through the airports,” a Metropolitan Washington Airports Authority spokesman said on Thursday.

The Washington disruption on Wednesday, in which four people died, followed reports of unruly Trump supporters on flights into the Washington area and prompted a prominent union leader to call for them to be barred from commercial flights out of the region.

“The mob mentality behavior that took place on several flights to the D.C. area … was unacceptable and threatened the safety and security of every single person on board,” Sara Nelson, president of the Association of Flight Attendants-CWA (AFA) representing workers at 17 airlines, said in a statement late on Wednesday.

“Acts against our democracy, our government and the freedom we claim as Americans must disqualify these individuals from the freedom of flight,” she said.

United Airlines, American Airlines and Delta Air Lines said they were working closely with law enforcement agencies to keep people safe and would continue enforcing strict mask policies on flights.

Some flight attendants have expressed concern that protesters would spread COVID-19.

United and American also increased staffing at the area’s airports and earlier this week moved their crews away from downtown D.C. hotels.

Southwest Airlines said it was monitoring events and expected customers to adhere to applicable laws, federal air regulations and crew member instructions while traveling.

The Transportation Security Administration, which has authority over US travel security, said late Wednesday it had multiple layers of security in place without providing details.