At least three associations are expressing support for Brussels following Tuesday's terrorist attacks while at the same time encouraging measures to increase transportation safety.

Two explosions at Brussels Airport and another at a nearby metro station killed at least 31 people, according to news reports. The fact mass transportation systems were targeted was not lost on many observers, including the U.S. Travel Association.

"The fact that travel and transit nodes were the object of this violence naturally has our full attention," USTA CEO Roger Dow said. "Those who would do harm to the Western world are a deranged minority and should be treated as such—our travel security policies should work to identify and separate them from the pool of legitimate travelers so that law enforcement can focus their full resources on bad actors and prosecute them as vigorously as possible."

In his statement, Dow appeared to take a swipe at Republican presidential frontrunner Donald Trump, who has suggested the U.S. temporarily ban Muslims from entering the country. However, Dow did not mention Trump or any other presidential candidate by name.

"Words matter and move markets," Dow said. "It's just not realistic to ban religions, order war crimes and end international travel. Bombast and bluster are no substitutes for thoughtful leadership."

Global Business Travel Association board President Christle Johnson didn't discuss policy in her statement, instead saying the association was "deeply saddened" by the attack.

"Dozens of innocent people were killed in an attack that was more than just an attack on an airport and a metro system, but an attack on all of us and our way of life.

American Public Transportation Association CEO Michael Melaniphy noted that as a result of the attacks, U.S. transit systems were expanding security measures as a precaution. He urged transit riders to report any unattended bags or suspicious behavior to security personnel.

"If you see something, say something," Melaniphy said. "We all need to work together to make sure that our public transit systems are as safe and secure as possible."

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