The deadly attacks happened more than 5,500 miles away.
But on Tuesday, police, transit agencies and airports from New York to San Francisc reacted to the terrorist-inspired explosions at the airport and subway in Brussels where 31 were killed and 212 injured.
All flights to Belgium were canceled, at least through Wednesday. None of the Bay Area airports have direct connections there, but travelers were bound to get stranded at their connection points, according to San Francisco International Airport spokesman Doug Yakel.
Airport employees were working with the TSA to see if any new security measures were warranted. For now, Yekel said, passengers can expect "heightened vigilance" in the form of police patrols around the airport. K-9 units officers were visible at Terminal 1 on Tuesday, as passengers reacted to the tragic news.
French citizen Thierry Fautier heard about the attack en route to San Francisco.
The Los Altos man and his wife were at the Bataclan Theatre in Paris Monday, paying respects to the victims of November’s terror attack.
"I think liberty’s at stake," Fautier said. "The U.S. knew that on Sept. 11. The same thing happens now in Europe and I think Europe has to be strong and make it tougher and tougher for those people."
Michael Iarocci and Debarati Sanyal, just off a plane from Paris, said they fear the attacks will ramp up fear in Europe and the U.S.
"The security is going to get stronger," Sanyal said. The Berkeley resident also expressed concern about increased "fear of foreigners" and "possibly more xenophobia."
But many vowed not to be deterred.
"Even though the news is sad, " said Aicha Khalfi, who was on her way to Paris, "we need to keep moving on."
Others, like Lanny Cohen of New York, suggested caution.
"We can succumb to it or we can be as prudent as we can possibly be, and just try to avoid situations that are more potentially at risk," he said.
San Francisco police spokesman Albie Esparza said police increased patrols around public transit and the airport in the wake of the attacks.
Transit officials don’t want to take any chances even though there has been no credible threat to local transit agencies. Security is being stepped up out of an abundance of caution.
Counter terrorism expert and UC Berkeley professor Bruce Newsome, however, believes the systems are still vulnerable.
“They aren’t secure,” he said. “We can’t lock down public transportation system or we would not have transportation systems.
That’s why even with no credible threat to the U.S., BART increased vigilance, stationing extra police officers at several stations. Officials also asked riders to "say something" if they "see something."
"We don’t discuss the specifics of how many people are out there and where they are, but its safe to say that we have stepped up our efforts," said BART spokesman Jim Alison.
Notices posted at Jack London Square in Oakland asked Amtrak passengers to stay alert and the San Francisco Municipal Transportation Agency took a similar approach with Muni.
Spokesman Paul Rose told NBC Bay Area via email that the agency is teaming up with the city's police department to increase police patrols throughout the system.
“Transit fare inspectors and security personnel will also be on patrol throughout the city to ensure that our riders and employees are as safe as possible," Rose said.
Passenger La Tiagra Harris, however, admitted that she still feels vulnerable.
"I am kind of scared," she said.
In the South Bay, the extra caution was put into place, too. the Santa Clara County Sheriff's Office and the VTA were also on heightened alert, the transit agency said, adding that employees recently completed an "all hazards" training.
According to Newsome, security measures need to be bolstered after any major attacks -- even if they occur on the other side of the world.
"Well, that is perfectly sensible because Jihadis are known to copy attacks," he said. "They do like to pile on the pressure by copying attacks that they have seen to have worked in other places."