The two suspects in the Boston Marathon bombing are brothers of Chechen ethnicity, Tamerlan and Dzhokhar Tsarnaev. According to the Djohar Tsarnaev vkontakte page (vk.com/id160300242, the Russian Facebook), he spoke English, Russian, and Chechen and was a Muslim. His brother Tamerlan Tsarnaev was an amateur boxer; in a 2010 college profile he said he was Muslim and was "very religious" (link). And:

If he wins enough fights there, Tamerlan says he could be selected for the US Olympic team and be naturalized American. Unless his native Chechnya becomes independent, Tamerlan says he would rather compete for the United States than for Russia.

The exact route the brothers took to the U.S. isn't clear, but their whole family appears to have applied for asylum around a decade ago, and they appear to have arrived in the U.S. from Dagestan, Kyrgyzstan, Kazakhstan, or another region near Russia.

Obviously, only a small number of such immigrants will harbor the same sentiments that led to the bombings. And, it doesn't appear that their immigration status was such that they would have been eligible for comprehensive immigration reform.

However, if they had applied for the amnesty being pushed by Barack Obama, Marco Rubio, and others, they would have almost certainly been waved right through. It would take approximately a decade to do FBI-quality background checks on 10 million illegal aliens, and those pushing amnesty have no plan to be so thorough. Almost all of those who'd be legalized under amnesty won't be terrorists or spies, but some very small percentage will be and discovering them isn't very likely due to the superficial background checks that will be performed.

UPDATE: The brothers' paternal aunt - now living in Toronto - claims she was a lawyer in her home country and says:

"All you’re showing is just the footage, two guys are walking. And I found it strange, Tamerlan is walking in the front, Dzhokhar is in the back. Why wouldn’t they come together? Just, you know, together as brothers? As I used to know them... I’m suspicious that this was staged! The picture was staged... [asked who staged it] Who ever needs this! ...Whoever is looking for those who need to be blamed for these attacks… Who is interested in this case? When you are blowing up people and you want to bring attention to something for some purpose, you know, you do that math... If you have evidence, if FBI has evidence, why are you asking question, do I believe they could do this? If they have done this, I have to believe. I would just know that they have done it... What do you think, that I would believe easily anything that my government, their government would throw me into my face? ...I don’t trust FBI, I don’t trust any agencies. Show me evidence... There’s a lot of material shown when those twin [towers at the World Trade Center] went off, with the suspicion of who had done that."

The brothers' father - who lives in Russia - says:

"I learned about the incident from TV. My opinion is: the special services have framed my children, because they are practicing Muslims. Why did they kill Tamerlan? He was supposed to be caught alive. The younger is on the run now. He was a sophomore at a medical school in the U.S. We expected him to come home for vacation. Now I don't know what's going to happen. Tell you once again: I believe special services have framed my children."

He also supposedly claimed that "all hell will break loose" if the younger brother isn't taken alive.

Bear in mind that some small number of people legalized under the Gang of Eight comprehensive immigration reform plan will have the same mindsets as the two brothers and their father and aunt.

UPDATE 2: Politico and CNN report that at the request of an unnamed foreign government, the FBI interviewed the elder brother about possible links to terrorist organizations and they found nothing "derogatory".

Also, the brothers' mother says:

Tamerlan was counseled by FBI for three, five years... They knew what my son was doing. They knew what actions and what sites on the Internet he was going. How could this happen? How could they - they were following every step of him and they’re telling today this is a terrorist act."

See immigration terrorism for extensive coverage of the issue going back years.