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."

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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.