Police in some of the nation's largest cities have stepped up security in the wake of two explosions that rocked the Boston Marathon.
"We've stepping up security at hotels and other prominent locations in the city through deployment of the NYPD's critical response vehicles until is more about the explosion is learned," Deputy Commissioner of the New York Police Department Paul J Browne told CNBC.
Mayor Michael Bloomberg told New Yorkers that security is being beefed up in both visible and covert ways. "We have 1,000 members of the NYPD assigned to counter-terrorism duties, and they - along with the entire NYPD and the investments we have made in counter-terrorism infrastructure - are being fully mobilized to protect our city," he said in a statement.
Security was also increased in Washington, D.C., at Pennsylvania Avenue in front of the White House, officials told NBC News. People with hard passes can still access the White House, but the public has been pushed into Lafayette Park and beyond. The Secret Service has put up yellow police tape to keep people away from the front of the White House.
Also, the Metropolitan Police Department in DC confirms to NBC News that the police have "heightened security" in DC. Police at the United States Capitol were taking additional precautions as well, according to a spokesperson.
Organizers from the London marathon — set to take place this Sunday — announced the race will still go on, though British police are in the process of reviewing security plans.
"A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon," Metropolitan Police Chief Superintendent Julia Pendry said.
Meanwhile, other cities announced they would not step up security until more is known about the incident. Police in Atlanta, Miami, Dallas, Houston, and Oklahoma City have all announced they are not making any adjustments, some because they do not have the resources, other because no major events were being held.
It is unclear what caused the explosion that killed two and injured dozens of runners and onlookers near the finish line of the marathon.
Additional reporting by NBC News' Stacey Klein and Mike Viqueira