The Official Story is the least likely explanation

"Questions surrounding 2011 triple murder point to government cover-up in Boston Marathon bombing":

"There has been no serious attempt to explain these staggering contradictions and anomalies. Instead, the same red herring used to cover up the role of US intelligence agencies in allowing the 9/11 attacks to take place—failure to “connect the dots” or “communicate”—has been trundled out once again before an incredulous public.

The official story—upheld by the Times—is the least likely explanation for the Boston bombing, an event that became the pretext for an unprecedented military-police lockdown of a major American city.

The most plausible explanation is that Tamerlan Tsarnaev was a US intelligence asset working with the FBI and/or other agencies. He was protected because he was deemed useful, perhaps in furthering US covert operations in the North Caucasus or other criminal US imperialist operations. Chechen Islamist terrorists make up a significant contingent of the foreign forces funneled by the CIA into Syria to help wage the current US-backed war for regime-change."

This CBC report is coherent only if you strip away the American propaganda assumptions and realize that both Tamerlan Tsarnaev and William Plotnikov were working for the CIA, probably with the dual purpose of investigating Islamist networks and causing trouble in Russia.  Todashev was murdered and his girlfriend is being quickly deported to cover up their knowledge of Tamerlan's government connections, the Waltham murders were given no real police investigation as Tamerlan was under CIA protection, the Russian repeated warnings about Tamerlan were ignored as Tamerlan was working for the Americans, and Tamerlan's trip to Dagestan and the Boston bombing were both official American government operations.  Nothing else makes any sense.