But there are some good reasons to hold off on drawing any conclusions from the fact that "Inspire" wrote about pressure cooker bombs.
Recall that the magazine was published in English, available for free online and widely covered in Western media. The issue was downloaded over 5,000 times on the jihadist forum Taawhed, more popular than any other issue since then, suggesting that a number of the downloaders were merely following their curiosity. The issue showed up on Western download sites more commonly used to distribute pirated music and movies, where it was passed around as a novelty item. It appears, in other words, to have flown to the far corners of the Web, where just about anyone could have picked it up and read all about pressure-cooker bombs.
And "Inspire" was far from the first extremist publication to distribute instructions for making pressure-cooker bombs. Yair Rosenberg of Tablet Magazine points out on Twitter that "The Anarchist's Cookbook," published in 1971, also included information on how to make them. The book appears to have provided the necessary instructions for at least one such bombing, in 1976 at Grand Central Station. In 1973, police had discovered a similar device in the New York Port Authority building.
Today, there appears to be a miniature subculture of Americans building small pressure-cooker bombs for the exclusive purpose of detonating them harmlessly in empty fields and posting video of the explosion to YouTube.
None of this is to dismiss the possibility that al-Qaeda or any other group could ultimately be connected to the Boston Marathon bombings. But it's worth keeping in mind that the June 2010 issue of al-Qaeda's "Inspire" was not exactly publishing privileged information when it discussed the horrific potential of gluing nails to the inside of a pressure cooker and placing it in a populated area.