Pressure cooker bombs are commonly used in Afghanistan, Pakistan, India and Nepal. It's easier to build bombs of that sort in the US rather than high explosive ones typically used by Al Quaeda. That's mainly due to the difficulty obtaining high explosives in the US, let alone importing them. The 2010 failed Times Square bomb was a pressure cooker.
I have no idea who did Boston. I also don't underestimate Al Queda or anyone else by thinking they won't change tactics if necessary, including staying mum until they perceive it timely to reveal themselves.