Security is being tightened at landmarks, government buildings, transit hubs and sporting events around the U.S. and the world following the explosions at the Boston Marathon.
In New York, authorities deployed critical response teams-- highly visible patrol units that move in packs with lights and sirens, -- along with more than 1,000 counterterrorism officers. Highly trafficked areas like the Empire State building, the United Nations and the World Trade Center site were being especially monitored.
At the White House, the Secret Service expanded its security perimeter after the attacks, shutting down Pennsylvania Avenue and cordoning off the area with yellow police tape. Several Secret Service patrol cars have blocked off entry points, although the White House was not on lockdown and tourists and other onlookers were still allowed in the park across the street.
And the worries are reverberating across the Atlantic. An already robust security operation is being beefed up for Wednesday's funeral of former Prime Minister Margaret Thatcher in Britain, where police are also reviewing security plans for Sunday's London Marathon.