CAMBRIDGE, Mass. — National Security Agency leaker Edward Snowden says the Boston Marathon bombings are an example of how threats can be missed even under blanket surveillance. Snowden, who leaked millions of documents to journalists, talked to Harvard Law professor Lawrence Lessig on Monday online from Russia, where he is in exile. The Boston Globe reports that Snowden referenced the suspected marathon bombers, saying “we knew who these guys were” but “we didn’t follow up or watch these guys.” Dzhokhar Tsarnaev is awaiting trial for April 2013 explosions that killed three people.