The Daily Beast's Eli Lake explains why the Tsarnaevs didn't elicit stronger responses from the FBI after they were tipped off by the Russian intelligence service, the FSB:
“The FSB is mad at a lot of Chechens,” said Michael Hayden, the former director of the CIA and the National Security Agency, who noted that he did not know at this point whether there was enough intelligence to warrant a full-scale investigation into Tsarnaev in 2011. “Not all of them are terrorists, and even fewer of them are dangerous to the United States.”
“When stuff like this happens, we did what we did we called the long pause,” said Hayden. “You get with your staff, you say it’s these two Chechen idiots, you see what’s in the database. You collect so much stuff, then you go and explain it eventually to Congress.”...
“We’ve all been through this,” said Hayden, who was director of the NSA on 9/11, about the criticism the FBI is now receiving. “People like me view this as the FBI’s turn.”