As Boston mourns the three victims killed during the marathon amid an outpouring of support for the survivors, the U.S. remains at a heightened state of security.
The saying, "If you see something, say something," has taken on a new meaning since Monday's twin bombings at the Boston Marathon also injured more than 170 people. Law enforcement is keeping anything but a low profile reacting to numerous security scares.
"I am not in the hope for the best business. I'm in the plan for the worst business," said Chief of the Los Angeles Police Department Charlie Beck.
COMPLETE COVERAGE: Boston Marathon Explosion
Emergency operators coped with a rash of "suspicious packages" called in Tuesday from coast-to-coast.
In downtown Seattle, streets were shut down because of a suspicious backpack left on a sidewalk. The bomb squad sent in a robot that pulled out a harmless hair dryer.
At Logan Airport, just miles away from Monday's attack, United Flight 636 en route to Chicago was delayed because the flight crew requested that customers and their bags be rescreened before departure. Two men were removed from the flight for "suspicious activity" then allowed to return to the plane, the Boston Globe reported.
At New York's LaGuardia Airport a suspicious item forced an entire terminal to evacuate. Ron Marsico of the Port Authority of New York and New Jersey, which runs the airport, says wires protruding from a fluorescent light fixture led someone to call authorities Tuesday morning, the Associated Press reported.
And in New Jersey, an unattended backpack shut down a PATH commuter train in Newark.
The New York City Police Department fielded 77 reports of suspicious packages in less than 24 hours. That's three times the norm, according to Police Commissioner Raymond Kelly. Kelly tried to calm the nerves of New Yorkers, insisting there have been "no specific threats" made against the city.
"You can't let people who are doing things like this get to you. Because then they win," commuter Carol Rodgers said before catching a train at Penn Station in Manhattan. "It's always better that you try and carry on and take the necessary precautions and continue with your daily lives. You can't let terrorists win."
Washington, D.C., remained vigilant on Tuesday keeping yellow police tape around the public sidewalk in front of the White House.
The epicenter of jitters remains in Boston, the site of the first fatal terrorist attack on U.S. soil since 9/11.
The number of Massachusetts National Guardsmen helping out in Boston was increased to 1,000 early Tuesday morning. Initially there were 460 Guardsmen who had already been assigned to the race prior to the blast to provide logistical support for the event.
Late Tuesday night, a candlelight vigil was held for 8-year-old Martin Richard near his Dorchester, Mass., home. The boy was killed by a bomb as he waited for his father to finish the marathon. Police and bomb-sniffing dogs secured the area around Garvey Park.
Security will be as tight as possible Thursday when President Obama visits Boston for an interfaith service.
ABC News Radio contributed to this report.