-- The U.S. State Department has issued a travel warning to Americans for the entirety of Europe in the wake of the terror attacks in Brussels citing the threat of "near-term" attacks throughout the continent.
"Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants, and transportation," said the alert, which expires on June 20, 2016.
"U.S. citizens should exercise vigilance when in public places or using mass transportation."
Americans were told to take particular caution "during religious holidays and at large festivals or events," the alert said.
Johnson said the department is concerned that radicalized individuals or small groups could carry out an attack within the U.S. "with little warning."
TSA is working with state and local law enforcement, airport authorities, and the aviation industry in order to augment the new security measures, according to Johnson.
Local law enforcement and airport authorities are responsible for securing individual airports around the country.
He also said that since the Paris attacks in November, federal authorities have enhanced information sharing about potential terrorist threats with France and Belgium.
Belgian citizens do not need a visa to travel to the U.S., but Johnson pointed out that all travelers are vetted against the U.S. Terrorist Screening Database.
"We continually evaluate whether more screening is necessary, particularly in light of today’s attacks," he said.
Johnson also said that his department is taking "a number of other security measures" that will not be disclosed to the public.