Even al-Qaida gloats about what's possible under U.S. gun laws. In June 2011, a senior al-Qaida operative, Adam Gadahn, released a video message rallying people to take advantage of opportunities those laws provide.
"America is absolutely awash with easily obtainable firearms," Gadahn says, explaining that "you can go down to a gun show at the local convention center" and buy a gun without a background check.
Then a faint smile crosses Gadahn's face. "So what are you waiting for?" he asks.
Investigators in protective suits examine material on Boylston Street in Boston on April 18, three days after the deadly bombings. The explosive devices were relatively simple to make and law enforcement officials come across them on a regular basis, officials say.
As investigators look into the Boston Marathon bombings, one crucial question is whether the suspects, brothers Tamerlan and Dzhokhar Tsarnaev, acted alone or had help. The clues might be found in the bombs used.
From what is now known, it appears the brothers assembled a whole arsenal of explosives. Watertown Police Chief Edward Deveau told CNN last weekend that the suspects had at least six bombs, including the two used in the attack and one thrown at police during a shootout.
Canadian authorities have disrupted an alleged plot that targeted a passenger train line running between New York and Toronto, the Royal Canadian Mounted Police announced Monday. The plan involved derailing a train, but police officials would give few details about the plot at a news conference this afternoon.
But they did say they believe the suspects received support or help from al-Qaida.
Osama bin Laden's son-in-law and former al-Qaida spokesman Sulaiman Abu Ghaith is expected to appear in a New York courtroom Monday afternoon.
Abu Ghaith was captured by U.S. officials in February, and his arrest is considered important not just because he was so close to bin Laden, but also because the Obama administration has decided to try him in a federal court instead of using a military commission at Guantanamo Bay, Cuba.