You have to start with the target(s). The targets were civilians, first responders and LEOs at a public event. In general terms, people that target civilians and non specific government employees are usually making a political statement. If the political statement was domestic in nature, the target would be specific (i.e. OKC Federal Building, Pentagon, Representative, Senator, etc). Since the target wasn't specific, we can presume that the group is likely international in nature whose goal is likely to be target "America".
In support, we look at other international terror attacks:
In 2010, a loan actor, trained in Pakistan, attempted to plant a car bomb in NYC Times Square. His goal was "to kill Americans."
2010 Times Square car bombing attempt - Wikipedia, the free encyclopedia
In 2007, four terrorists used suicide bombs to target UK civilians on public transportation.
7 July 2005 London bombings - Wikipedia, the free encyclopedia
The common thread is that they don't target anything more specific than the general population. They also used a bomb style that is specifically promoted by Al Qaeda. If you compare Boston to NYC and UK bombings, they used bombs previously used by Al Qaeda, they target civilians and they attempted to kill as many people as possible. Granted, it is only speculation, but it is reasonable to believe this will be an attack that is attributed to Al Qaeda or an affiliate.