Although authorities said there was no known threat to Chicago Tuesday, they also were urging local to be vigilant, especially in crowded places.
At a Cubs game Tuesday night, there was heightened security. A bomb-sniffing dog swept the field before fans arrived, some trash cans were removed from the park and police were out in force.
"I've seen groups of police officers walking together, which is unusual. Usually you'll see a few scattered, but they seem to be teaming up today," Mike Dahms
Safety experts say the areas near train stations, for example, are the kinds of areas where people should be alert because hundreds of people can pass through them at a time.
Former police commander and security consultant Neil Sullivan has been a key leader in setting up major event security in Chicago for the past decade - from sports championships to the Obama victory celebration to NATO.
All of them required months of planning, defined security footprints, constant communication, even things like removing trash cans and newsstands.
"The only absolute is, there are no absolutes in this business," he said.
And everywhere there is a call for vigilance, and a hunger for information. Are important clues emerging?
A professor of political science at the University of Chicago Robert Pape has spent the last ten years studying terrorism particularly suicide terrorists.
The nature of the explosive used in Boston, a so-called pressure cooker bomb and the absence of any claimed responsibility leads him to believe that yesterday's carnage was more in keeping with "lone wolf" terrorism.
"This looks even more like a deliberate effort to maim - not so much as to create dead people, but to injure and call attention to some cause," Pape said. "This is pointing in the direction of fewer people involved. It's less likely than an international group. It doesn't fit the profile of international groups."
Police swept the area where the bombs exploded early Monday morning and an hour before the marathon began which suggests a fairly narrow window during which the bombs were planted.
And with so many cameras- professional, amateur and surveillance recording this area, investigators appealed to the public today - give us whatever you've got.
Officials say being aware is part of the battle, even though security has been beefed up.
TSA agents were patrolling near Metra stations downtown. In northwest suburban Mt. Prospect, squad cars monitoring were monitoring the Metra stop.
Many commuters had the Boston bombings on their minds as they headed to work Tuesday.
"You have to keep going with your life, but the more security the better," said commuter Reggie Ozoa.
"I'd rather feel safe than anything else," said Teresa Day, also a commuter.
Bomb-sniffing dogs could be spotted hounding trash cans in Union Station Monday night. At the CTA State and Lake Red Line stop, Chicago police were making their presence known.
"It makes me feel better. It's kind of scary," said rider Nick Koeppen.
Local law enforcement officials, along with the Office of Emergency Management, say there are no known threats to the Chicago area, but they are in communication with other agencies.
Former police superintendent and ABC7 Public Safety Expert Jody Weis says Chicago officials are working closely with a web of law enforcement and investigators nationwide.
The best advice from experts: Don't be scared, but be alert.
The biggest thing to remember they say is don't be afraid to say something if you see something suspicious. Authorities say they would rather have several false alarms than one catastrophe.