Default assumptions should be based on likelihoods, derived from recent history. If most recent terrorist attacks were domestic extremist groups, there might be a basis for such speculation, but multiple explosions, a short time apart? Not aimed at a government building or abortion clinic? If there is an appropriate default assumption, it is al-Qaeda or other Islamist groups -- not a domestic right-wing group.
But that's why it is called National Palestinian Radio.
UPDATE: Hardly conclusive, but this April 16, 2013 Telegraph report indicates that pressure cooker bombs (now believed to be what was used in Boston) were described in the first issue of al-Qaeda's English-language publication for aspiring jihadis:
The recipe – along with a rationale for post-9/11 terror – was printed three years ago in al-Qaeda’s English-language promotional online magazine, Inspire.
In an article, it instructed readers on how, as its headline writers put it, to “Make a bomb in the kitchen of your Mom”.
It gave the types of explosive, timers and other ingredients needed – along with, it said, a pressure cooker.