But at least it looks like a domestic incident, not international terrorism.
- No one has claimed credit
- The bombs were home-made, and half the bombs did not activate
- They struck a great city but not an internationally famous one (Washington, New York, Los Angeles, Chicago, Houston.)
When it comes to domestic terrorism, the groups are more afraid of the FBI. They can brag internally, without needing international publicity. International terrorism get minimal if any benefit if they don't publicly take credit (which is why so often you get multiple false claims of credit).
International terrorists have lots of opportunity to practice and gain professional bomb making ingredients. Domestic ones do not have that opportunity. They have to learn on the internet, with as little practice as possible to avoid attracting the attention of local police.
Boston is a great city, but it #21 in population. El Paso Texas has more people (Source). It is not an international center, like Washington, New York, or famous (from movies) as Los Angeles. It's a great city but not a prime target.
For all these reasons, I think the Boston Bombing was done by domestic (probably non-islamic) terrorists.