Security tight around U.S. after Boston blasts

A Boston police officer stands near the scene of a twin bombing at the Boston Marathon in Boston, Mass., April 16, 2013. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence.

Read more: Boston blasts prompt major cities to step up security

A U.S. Capitol Police K-9 team works at the entrance of the Capitol South Metro station during rush hour in Washington D.C., April 16, 2013. Police and other law enforcement agencies in Washington and across the country continue to operate at a heightened state of security after yesterday's Boston Marathon bombings.

Seattle Police Dept. vehicles block off a street while remote controlled vehicles are used to examine a suspicious backpack in downtown Seattle, April 16, 2013. The backpack was found to be harmless, but in light of Monday's explosions at the Boston Marathon, authorities in Seattle and other cities increased security patrols and heightened the level of alertness for any possible threat to public safety.

A law enforcement officer stands post at the U.S. Capitol in Washington D.C., April 15, 2013.

SWAT team members and special police units assemble in the Boston Common in Boston, Mass., April 16, 2013.

A bystander takes a photo from a window as Seattle Police Dept. robots are used to remove an item from a suspicious backpack in downtown Seattle, April 16, 2013. The backpack was found to be harmless, but in light of Monday's explosions at the Boston Marathon, authorities in Seattle and other cities increased security patrols and heightened the level of alertness for any possible threat to public safety.

A heavily armed Boston police officer stands blocks away from the scene of a twin bombing at the Boston Marathon, April 16, 2013.

Amtrak Explosive Detection K-9 officer Kenneth Wolf and his dog Teddy patrol the station tunnels at Union Station in Los Angeles, April 15, 2013. The city increased security following bomb explosions in Boston that killed two people and injured more than 80 near the crowded finish line of the Boston Marathon.

A heavily armed Boston police officer stands blocks away from the scene of a twin bombing at the Boston Marathon, April 16, 2013.

A National Guard soldier checks identification at a roadblock near the scene of a twin bombing at the Boston Marathon, April 16, 2013.

New York Police Department officers inspect bags at a subway entrance during the morning commute in Grand Central Terminal in New York City, April 16, 2013. The city announced it has continued increased security one day after the Boston Marathon bombings.

Seattle Police K9 officer Craig Williamson stands with his explosives detection dog, "Dennis" as he patrols downtown Seattle in reaction to explosions at the Boston Marathon finish line earlier in the day, April 15, 2013. A Seattle Police blog posting said that although there was no indication of a direct threat to Seattle, officers would be increasing patrols and activity around the city.

Metropolitan Transit Authority (MTA) police officers keep watch during the morning commute in Grand Central Terminal in New York City, April 16, 2013.

A National Guard soldier keeps watch during the morning commute in Grand Central Terminal in New York City, April 16, 2013.

New York Police Department officers inspect bags at a subway entrance during the morning commute in Grand Central Terminal in New York City, April 16, 2013.

A New Jersey State Police helicopter flies by the construction site of One World Trade Center in Manhattan seen from The Heights neighborhood of Jersey City, N.J., April 15, 2013. New York is securing its city in the wake of explosions near the finish line at the Boston Marathon.

A uniformed Secret Service officer pulls tape across the road to close Pennsylvania Avenue near the White House, April 15, 2013 in Washington.

A Los Angeles Sheriffs Counter Terrorism Unit patrols at Union Station as Los Angeles Dodgers fans commute in Los Angeles, April 15, 2013. The city increased security following bomb explosions in Boston that killed two people and injured more than 80 near the crowded finish line of the Boston Marathon.

Uniformed Division Secret Service officers stand watch on the sidewalk in front of the White House looking north from Pennsylvania Avenue during heightened security following the explosions in Boston, April 15, 2013.

A Los Angeles Sheriffs Counter Terrorism Unit officer carries an AR-15 assault rifle, as he patrols the entrance to Union Station in Los Angeles, April 15, 2013. The city increased security following bomb explosions in Boston that killed two people and injured more than 80 near the crowded finish line of the Boston Marathon.

Los Angeles Amtrak Police officers arrest a suspect outside Union Station in Los Angeles, April 15, 2013. The city increased security following bomb explosions in Boston that killed two people and injured more than 80 near the crowded finish line of the Boston Marathon.

Amtrak Explosive Detection K-9 officer Kenneth Wolf and his dog Teddy ask a passenger to inspect his luggage before boarding a departing train at Union Station in Los Angeles, April 15, 2013.

Los Angeles Amtrak Police officers arrest a man outside Union Station in Los Angeles Monday, April 15, 2013. The city increased security following bomb explosions in Boston that killed two people and injured more than 80 near the crowded finish line of the Boston Marathon.

People stand behind police tape after law enforcement closed down Pennsylvania Avenue in front of the White House in Washington D.C., April 15, 2013.

A Los Angeles Sheriffs Counter Terrorism Unit deputy patrols Union Station before the start of a Dodgers game in Los Angeles, April 15, 2013. The city increased security following bomb explosions in Boston that killed two people and injured more than 80 near the crowded finish line of the Boston Marathon.

A police officer patrols the area in front of the Barclays Center in New York, April 15, 2013, before a Brooklyn Nets NBA basketball game in the wake of the explosions at the Boston Marathon.

Two law enforcement officers carry a barricade to help close down Pennsylvania Avenue in front of the White House in Washington, D.C., April 15, 2013.

Amtrak police officers and their dogs watch as commuters walk through Penn Station in New York City, April 16, 2013.

Metropolitan Transit Authority (MTA) Canine Unit Police Officer Rozsay and dog Seren keep watch during the morning commute in Grand Central Terminal in New York City, April 16, 2013.