Boston bombs said to be made from pressure cookers packed with shards of metal, nails and ball bearings
In the wake of the attack, security was stepped up around the White House and across the country. Police massed at federal buildings and transit centers in the nation's capital, critical response teams deployed in New York City, and security officers with bomb-sniffing dogs spread through Chicago's Union Station.
Homeland Security Secretary Janet Napolitano urged Americans "to be vigilant and to listen to directions from state and local officials." But she said there was no evidence the bombings were part of a wider plot.
Pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 intelligence report by the FBI and the Homeland Security Department. One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the report said.
"Placed carefully, such devices provide little or no indication of an impending attack," the report said.
Investigators said they have not yet determined what was used to set off the Boston explosives. Typically, these bombs have an initiator, switch and explosive charge, according to a 2004 warning from Homeland Security.
"We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice," the FBI's DesLauriers said.
The Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any part in the Boston Marathon attack.
Al-Qaida's branch in Yemen gave a detailed description of how to make a bomb using a pressure cooker in a 2010 issue of Inspire, its English-language online publication aimed at would-be terrorists acting alone.
In a chapter titled "Make a bomb in the kitchen of your mom," it says "the pressurized cooker is the most effective method" for making a simple bomb, and it provides directions.
Naser Jason Abdo, a former U.S. soldier, was sentenced to life in prison last year after being convicted of planning to use a pair of bombs made from pressure cookers in an attack on a Texas restaurant frequented by soldiers from nearby Fort Hood. He was found with the Inspire article.
Investigators are also combing surveillance tapes from businesses around the finish line and asking travelers at Boston's Logan Airport to share any photos or video that might help.
"This is probably one of the most photographed areas in the country yesterday," said Boston Police Commissioner Edward Davis. He said two security sweeps of the marathon route had been conducted before the bombing.
Boston police and firefighter unions announced a $50,000 reward for information leading to arrests.
Obama said officials do not know who carried out the attack or why — "whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual."
But he said "any time bombs are used to target innocent civilians, it is an act of terror." And he declared: "The American people refuse to be terrorized."
Sullivan reported from Washington. Associated Press writers Steve LeBlanc, Bridget Murphy, Rodrique Ngowi and Meghan Barr in Boston; Julie Pace and Lara Jakes in Washington; Paisley Dodds in London; Lee Keath in Cario; and Marilynn Marchione in Milwaukee contributed to this report along with researcher Randy Herschaft in New York.
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