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Residents in major U.S. cities are likely to see an increased police presence and other security measures as authorities try to find out who is behind Monday’s deadly Boston Marathon blasts.

With no one taking responsibility for the attack yet, and no known motive, authorities in Washington, D.C., and New York, as well as Los Angeles, Philadelphia, and elsewhere in the country said they are remaining vigilant.

Security was revved up near the White House, and Pennsylvania Avenue was closed to pedestrians. The area directly in front of the White House was cordoned off with yellow tape.

U.S. Capitol police said it had "increased visibility" in a number of areas. Staff and visitors to the Capitol can expect increased police presence and more frequent canine inspections.

New York police stepped up their numbers in the subways and at popular places like Times Square, midtown Manhattan, the World Trade Center, and the United Nations. New Jersey Transit said it was in a “heightened state of alert” in response to the explosions.

New York City Mayor Michael Bloomberg said in a statement that he will deploy 1,000 members of the NYPD assigned to counter-terrorism duties. The announcement comes as a 9/11 Memorial 5K run is scheduled for Sunday, which ends near Ground Zero.

Similarly, authorities in other major cities, including  Los Angeles, Philadelphia, Las Vegas, Detroit, and Atlanta said they too were closely watching for information about possible threats and may increase security.

Meanwhile, two security incidents affected flyers at New York's LaGuardia Airport and Boston's Logan Airport on Tuesday. A section of LaGuardia was evacuated for 15 minutes after a "suspicious package" was found. Authorities eventually gave the all clear signal and fliers were allowed to return to their terminal. At Logan, a flight was briefly pulled off the runway and a suspicious piece of luggage was examined, according to NBC News. Passengers were evacuated from the plane but were allowed to return after the situation was deemed safe.

Several sporting events were also  cancelled or postponed, including Monday night’s hockey game between the  Boston Bruins and Ottawa Senators . The NBA canceled Tuesday night’s game between the Boston Celtics and the Indiana Pacers.

President Obama on Tuesday called the Boston explosions, which killed three and injured more than 170,  a “heinous and cowardly act,” adding “The FBI is investigating it as an act of terror.” Obama said the authorities do not know who carried out the attack, but “we will bring them to justice.”

The explosions in Boston put authorities in other countries on alert, too. British police said they are reviewing their security plan for Sunday’s London Marathon, and Russian sports officials said they will increase security at upcoming sports events and at the Sochi 2014 Winter Olympic Games.

British officials also promised to step up security efforts for former Prime Minister Margaret Thatcher’s funeral on Wednesday.

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