Bombs went off at the Belgian capital's airport and on a metro train during the morning rush hour. At least 31 people were killed and dozens were injured.
Mayor Rahm Emanuel said while there is no immediate threat to Chicago, the city will be on alert to keep residents safe. He said in a statement Tuesday that the city of Chicago stands with the people of Brussels.
"While an ocean may separate us, Chicago and Brussels are united by common values of life, liberty, and the pursuit of happiness," Emanuel said.
Interim Police Superintendent John Escalante and the FBI said Tuesday there is no known threat to the city at this time in connection with the attacks, but he wants the department to be prepared and ready to safeguard critical infrastructure. Similar precautions were taken in New York City and in Washington, D.C. Local authorities said they're in contact with federal and state officials. Police urged anyone who observes suspicious activity in Chicago to call 911.
STATE DEPARTMENT ISSUES EUROPE TRAVEL ALERT
The State Department warned Americans about potential risks to travel in Europe. "Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants and transportation," it said in a statement. Officials advised U.S. citizens to "exercise vigilance" in public places or on mass transportation and to take "particular caution" during religious events or large events.
On Tuesday night at O'Hare, passengers bound for Europe were just learning of the State Department's warning.
"It's a concern. Of course it's a concern, but I think we decided not to cancel the trip. You're either in the wrong place at the wrong time, or you're not," said Susan Hancock.
"I think I feel more alert, but I'm not terribly anxious. You either travel or you don't," said Nancy Baglan.
Some airlines are waiving change and cancellation fees for certain European cities. But after the State Department alert, more airlines could adopt a similar policy for more destinations.
Everywhere around O'Hare Airport Tuesday there were signs of a stepped-up police presence, including bomb-sniffing dogs and increased foot patrols. Officials with the Department of Homeland Security scoured O'Hare's terminals Tuesday.
"We rely on Chicago police as we always do and they work several local and federal agencies and we're very comfortable with whatever protocols and procedures they have in place," Chicago Dept. of Aviation Commissioner Ginger Evans said.
CPD admits to adding both uniformed and plainclothes officers to the airports, but insists that "there is no current intelligence threat or nexus to Chicago concerning the Brussels incident."
For travelers going through O'Hare, the attacks in Belgium are disconcerting, but not cause for concern here at home.
"I'm not apprehensive about it. I don't like to make myself nervous," traveler Dieva Johnson said.
"Clearly it's a horrible story, but it doesn't change my feelings about getting on a plane. I think we've got good security practices," said Matt Phillips, another traveler.
Even for those traveling to Europe, the terrorist attack was seen with some degree of resignation and not as something that was going to disrupt any plans.
"It doesn't concern me that much. I trust in the European Union so I still trust in the government and our democracy," said Kai Hochstuhr.
"Life goes on. If it's going to happen, it's going to happen," said Jorgen Lekandeo.
Most passengers said they were willing to put up with additional security and longer lines if it will keep airports safe. Anyone traveling to Europe should check with their airlines to see if their flights are impacted.
Amtrak officials said they are partnering with local law enforcement to keep the railroad and train stations safe. Extra officers were deployed Tuesday.
Metra, CTA and Amtrak all advised their passengers that if they see something, say something.
Be vigilant & observant about suspicious activity. SEE something, SAY something. Metra Police 312-322-2800 or 911. pic.twitter.com/22V6RJ7WQ9— Metra (@Metra) March 22, 2016
DePaul University confirmed 17 students and a faculty member from DePaul University traveling in Belgium as part of a study abroad group are safe.
We are thankful to report all SPS students & faculty in Belgium for study abroad are safe from today?s attacks.Thoughts& prayers to Brussels— DePaul SPS (@DePaulSPS1) March 22, 2016
The group is staying in the city of Leuven and were headed to Brussels when they heard the news.
"We missed the train. We would have been in Brussels - on the way to Brussels - when that Metro bombing occurred. And thankfully we got to take a later train. We would have been stuck there all day and as a worst case, had we left earlier, we would have been in the same neighborhood of that bombing," said Joseph Schwieterman, a DePaul University professor.
The group is scheduled to come back to Chicago on Sunday, but they're not sure how the attack will impact their travel plans.
The Associated Press contributed to this report.