Amid questions about whether the FBI missed an opportunity to discover that one of the suspects in the Boston Marathon bombings may have become an extremist, law enforcement officials tell the New York Times they had no legal basis to monitor him in the months leading up to the attack. “We had an authorized purpose to look into someone based on the query we received,” an official said. “You can do a limited investigation based on that request.” FBI and intelligence officials will be forced to explain to the Senate Intelligence Committee in a classified briefing today the steps they took – and did not take – before and after Tamerlan Tsarnaev returned last July from a six-month trip to Russia. The exchange between the FBI and Russian authorities on Tsarnaev's potential links to extremist groups has cast a spotlight on a counterterrorism relationship that has endured even as diplomatic relations between the countries have gone through ups and downs. It also reflects what Daniel Benjamin, the State Department's former top counterterrorism official, called “a culture of wariness” between the two former cold war rivals.