WASHINGTON – Major U.S. transportation hubs were on alert on
Tuesday with police out in force, and part of Denver International Airport’s
main terminal was evacuated in response to a possible security threat there
after deadly suicide bombings in Brussels killed at least 30 people.

The evacuation area at Denver encompassed the west side of the
main terminal, levels five and six, but some flights were able to go on while
the possible threat was investigated, the airport said.

Vehicle traffic was being allowed to continue on the west side
of the Denver terminal, and the east side remained open to passengers, the
airport said. The nature of the possible security threat was unclear.

Major U.S. airports and other transportation facilities were put
on alert following attacks at Brussels airport and a subway station on Tuesday
for which Islamic State claimed responsibility.

Denver airport on its Twitter feed said that airline ticket
counters affected by the incident there included American Airlines, Aeroméxico,
Air Canada, Lufthansa, and British Airways. “Flight delays are
possible,” it said.

A Denver airport spokesman, Heath Montgomery, said earlier in
the day that no additional security measures were being taken there, though he
said airport officials remained in contact with the U.S. Department of Homeland
Security and Transportation Security Administration.

“Especially in light of the events today, we share a heightened
sense of awareness,” Montgomery said.

U.S. President Barack Obama and Democratic presidential election
contender Hillary Clinton vowed to do more to take on militants, while
Republican presidential front-runner Donald Trump called for tighter border
security and suggested U.S. intelligence services could use torture to head off
future attacks.

The Obama administration was expected to tighten security
measures at U.S. airports following the Brussels airport attack, which occurred
in a public hall outside of the security check area.

U.S. Representative William Keating of Massachusetts, the ranking
Democrat on a House subcommittee on terrorism, said the Brussels attacks
illustrate the difficulty of protecting “soft targets” outside
tightly controlled security cordons.

“We should learn from this that the targets aren’t going to
be just getting on the plane itself, but the airport in general,” Keating
said in a phone interview.

U.S. Homeland Security Secretary Jeh Johnson said the agency has
no intelligence that would point to a similar attack being plotted against the
United States.

Delta Air Lines Inc,
United Continental Holdings Inc and American Airlines Group Inc canceled or rerouted flights as a result of the attack.

Large
numbers of uniformed police officers and National Guard members in fatigues and
carrying long weapons patrolled New York’s John F. Kennedy International
Airport.