If this was a anti-government protester, he/she would have likely targeted a government building, like the IRS, Congress or some other symbol of Government.
If this is a foreign terrorist attack, then a high profile international event like the Boston Marathon would fit. There has been a lot of warnings that jahadists want to do acts of terrorism within the US. So this could be the start of that. A little too early to tell, but likely.
(sarcasm) As to immigration reform, what could possibly go wrong with opening up our boarders (/sarcasm)
“Patriot’s Day”, the Boston Marathon, and TWO explosions (which I’ve read the CIA has been doing with drone attacks) — sounds more like Jihadists than anti-government, environmental groups or even Occupy Wall Street. Pure speculation, of course.