Armed British police officers stand guard after Eurostar services were suspended on the Brussels route because of the attacks in Belgium, at St Pancras international railway station in London, Tuesday, March 22, 2016. Authorities in Europe and beyond have tightened security at airports, on subways, at the borders and on city streets after deadly attacks Tuesday on the Brussels airport and its subway system. (AP Photo/Matt Dunham)

Checkpoints outside of airport checkpoints necessary, security experts say

Terrorists directed by ISIS detonated bombs in Brussels that killed dozens of victims in an airport and metro system,.