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.
The tightly sealed pot makes easier-to-obtain but weaker explosives faster and stronger, amplifying the blast and the carnage.
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 Fort Hood. He was found with the Inspire article.
Investigators in the Boston bombing also are combing surveillance tapes from businesses around the finish line and asking travelers at 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.
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 Cairo; and Marilynn Marchione in Milwaukee contributed to this report along with investigative researcher Randy Herschaft in New York.