.. and why the FBI didn't investigate aggressively.
The Russian tip to the FBI about Tamerlan Tsarnaev two years before the Boston bombings was a result of his frequent visits to a mosque in Dagestan known for producing extremists . Tsarnaev came to the attention of Russian security forces after becoming a regular at the Salafist mosque during a 2011 visit. I seems here wasn’t enough time in 2011 to come to any conclusions about the extent of his involvement in Islamist extremism. While sources in Russia say Tsarnaev was never observed making contact with insurgent leaders, he very likely heard sermons that pushed "the idea that America and Israel are the axis of evil.
After receiving the tip, the FBI interviewed Tsarnaev and searched its databases before quickly closing the case; Tsarnaev returned to Dagestan for an extended trip the following year—a trip that has become a "key thread" in the bombing investigation. The FBI's 2011 moves are now being heavily scrutinized, but the FBI shouldn't be expected to aggressively pursue every tip from Russia's FSB security service.
The service has been criticized for being used as a tool to attack the Russian regime's political opponents, and there doesn't seem to have been any indication that Tsarnaev posed a threat to the US. "The FSB is mad at a lot of Chechens," says former CIA director Michael Hayden. "Not all of them are terrorists, and even fewer of them are dangerous to the United States."