Security was boosted across the United States after a double blast killed at least two people and injured more than 100 at the Boston Marathon Monday.
President Barack Obama ordered federal agencies to help police and local officials prevent any further attacks by increasing vigilance and catching the perpetrators of what White House staff described as an "act of terror."
"We're continuing to monitor and respond to the situation as it unfolds," Obama said in a brief statement.
"I've directed the full resources of the federal government to help state and local authorities protect our people, increase security around the United States as necessary, and investigate what happened."
Even before there was official word on whether the explosions were an act of terrorism, New York and other cities quickly took extra precautions at potential targets like ports, airports, sports venues, government offices, and major landmarks.
The Federal Aviation Authority briefly grounded all flights at Boston's Logan International Airport and imposed a no-fly zone over the blast site.
Meanwhile, the national rail service, Amtrak, boosted security at train stations and track crossings.
New York Mayor Michael Bloomberg said security was being stepped up at "strategic locations and critical infrastructure, including our subways."
"We have 1,000 members of the NYPD assigned to counterterrorism duties, and they -- along with the entire NYPD and the investments we have made in counterterrorism infrastructure -- are being fully mobilized to protect our city," Bloomberg said in a statement.
"As law enforcement authorities investigate today's explosions in Boston, I ask all New Yorkers to keep the victims and their families in your thoughts and prayers."
Police in the nation's capital of Washington, DC -- which like New York was a target of the September 11, 2001 terrorist attacks -- have also "heightened security," spokeswoman Saray Leon told AFP.
Los Angeles police chief Charlie Beck announced an increased police presence at "all scheduled sports events in the near future," beginning with the Dodgers-San Diego Padres baseball game on Monday evening.
The additional police presence would include deployment of bomb-squad personnel, dogs and other "precautions geared to preventing a similar event."
Security was also stepped up at train stations, government buildings, shopping centers and "places where people congregate," Los Angeles County Sheriff Lee Baca said.
Police in the West Coast city of San Francisco said they were on "heightened alert" and urged people to call 911 if they see anything suspicious "in light of the Boston marathon explosions."
Chicago police were "closely monitoring events as they unfold in Boston" but "are not aware of any additional threats" at this time, spokeswoman Melissa Stratton said.
Illinois Governor Pat Quinn ordered the state's public safety agencies to "stand ready to assist and remain vigilant in responding to and reporting suspicious activity" and urged members of the public to do the same.