Yes I believe the the Boston marathon bombing suspects are part of a larger terrorist group. There are a few reasons I believe this. First, the bombs used at the Boston marathon, although somewhat crude, did show a level of sophistication not usually seen in lone wolf terrorists. Also, the calm, casual way the bombers acted before, during and immediately after the bombings showed that they had likely trained, practiced this scenario many time before the actual event.
Think about it. If you were wanted, why would you put yourself in the public eye by robbing a 7-11. Someone in the higher-ups must have threatened them with their family's safety and told them that if they didn't go on this suicide mission by robbing a liquor store, then there would be consequences.
It is my belief that these two men acted alone in their planning and execution of the bombings. That is not to say that they were not influenced by the ideals of some terrorist groups or wanted to be a part of them in an official way. Given the fact that no terrorist group as of yet has come out and claimed responsibility, I think that bolsters the probability that these two coordinated and planned this on their own.
Additionally, if they were working with a terrorist group, why would they stay in the area and why would they need to rob a 7-11 to acquire money to flee the area. A well organized attack, funded by a terrorist group would likely have provided them with the means to escape the area immediately after the attacks took place.
Most official terrorist cells are under intense and coordinated surveillance efforts by cooperation between multiple countries after 9/11 - which is the main reason why there were no successful attack on US soil after that day. One can get the full list of (foiled) attacks here: http://politicalscience.Osu.Edu/faculty/jmueller/since.Html -- this is a good reading since some of the plots were quite comical at times.
Lone-wolf terrorism, on the other hand, has been proven quite successful, since it is practically impossible to detect. Bomb making instruction can be found easily on the internet, made from inconspicuous materials (fertilizer, pressurized cooker, nail, ball bearing? Quite inconspicuous i'd say) which can be bought from the internet too, and from multiple sources in small quantity at times. No (air) travel is necessary - the TSA, annoying as they might be, are quite good at what they do. As such, home grown terrorist are by far the hardest to detect.
All the evidence doesn't add up they've been framed. How did police know of bombing prior with k9. Why would they realize a hostage unharmed but kill an unaware officer. No 19 year old could outsmart the FBI and his backpack was grey. Not only that the government had previously questioned the older brother years back and found know evidence yet now say he had searched radical videos?
Many terrorists associate themselves with the ideology or methods of proven cells, without ever having met actual members. The crude bombs used in the attack, as well as the actions of the suspects afterwards seem to imply that they acted alone. Additionally, no known terrorist group has claimed credit for the bombing.