LOS ANGELES (KABC) -- Security was immediately heightened in cities around the country in the aftermath of two explosions that rocked the Boston Marathon Monday.
Police in New York, Chicago, Los Angeles and other major urban centers were increasing personnel as a precaution.
Homeland Security Secretary Janet Napolitano is directing her agency to provide "whatever assistance" necessary in the wake of two explosions near the finish line of the Boston Marathon.
Cellphone service was shut down in the Boston area to prevent any potential remote detonations of explosives, according to a law enforcement official.
The Los Angeles County Sheriff's Department opened an emergency operations center and ordered an increase in patrols at transit areas and other critical locations, including at Dodger Stadium for Monday night's baseball game.
Los Angeles Police Officer Bruce Borihanh said the LAPD "will also be increasing the security in and around the Dodger game scheduled for later this afternoon." The Dodgers will take on the San Diego Padres at 7 p.m.
Security was stepped up at Metro facilities in the Southalnd, including at Union Station, with additional officers and K-9 units, according to Marc Littman, a spokesman for the transportation agency.
Littman said there was no disruption to any service, but passengers will see more officers on patrol. As usual, Littman urged passengers to report any suspicious activities on buses and trains or at stations. As usual, Littman urged passengers to report any suspicious activities on buses and trains or at stations.
The Los Angeles Police Department said it was keeping close tabs on the Boston situation.
In Orange County, Sheriff Sandra Hutchens said deputies would "have an increased presence at local sporting/entertainment venues and other large gathering places."
Two bombs exploded near the finish line of the Boston Marathon on Monday, killing two people and injuring at least 23 others in a terrifying scene of broken glass, smoke and severed limbs, authorities said.
A third explosion was reported at the JFK Library more than an hour after the first two. No one was injured in the third explosion. Boston Police said it was not immediately clear if it was related to the first two, but they were treating it as if it were.
Authorities evacuated the Kennedy School of Government at Harvard just after 5 p.m. Eastern Time.
The Federal Aviation Administration established a no-fly zone over the site of two Boston explosions. The agency said in a notice issued Monday about an hour after the explosions that a no-fly zone with a 3.5-mile radius has been created over 811 Boylston Street. That was later reduced to a 2-mile radius.
The zone is limited to flights under 3,000 feet in altitude, which is lower than most airliners would fly except when taking off or landing. The notice says the no-fly zone is effective immediately and will remain in effect until further notice. Pilots planning flights were urged to call their local flight service station.
New York Police deployed critical response teams around the city. "The NYPD has stepped up security at strategic locations and critical infrastructure, including our subways. Some of the security steps we are taking may be noticeable .... and others will not be," New York Mayor Michael Bloomberg said in a statement.
California emergency management officials activated their statewide threat assessment system, which was established following the Sept. 11, 2001 World Trade Center bombings.
Officials in San Francisco, Los Angeles, Orange County, San Diego and Sacramento were reviewing information from federal authorities for possible threats, said Kelly Huston, assistant secretary of the California Emergency Management System.
San Francisco police Officer Albie Esparza says patrol officers are being asked to be extra vigilant around critical infrastructure and areas where large crowds gather. The department is also going to rethink security for the upcoming San Francisco Marathon in June and the Bay to Breakers foot race in May.
San Jose police say the department is not modifying its field deployment of officers or security at any critical infrastructure sites, like the airport.
Police in Washington, San Diego, Vegas and Atlanta were monitoring events closely and assessing potential increases in security measures. Agencies were also stepping up social media response, telling the public via Twitter and Facebook to report suspicious activity to the police.
Police in Britain were reviewing security plans for the upcoming London Marathon scheduled for Sunday. The London Marathon is the next major international marathon. The London Marathon is a hugely popular race. Last year, some 37,500 athletes competed, with many more watching the springtime event. London has long been considered a top target for international terrorists, with the government saying the threat level is "substantial." In 2005, a series of suicide attacks on the public transport system in the British capital killed 52 people.
Boston Police meanwhile urged citizens to stay indoors and not to congregate in large groups Monday while the investigation continued.
The Associated Press and City News Service contributed to this report.
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