The FBI has released an internal memo from a 2011 interview with the man who would go on to become one of the Boston Marathon bombers, Tamerlan Tsarnaev. The memo references an odd encounter Tsarnaev told the agents. This story was first reported in the Boston Globe.  

In April 2011, FBI agents visited Tamerlan Tsarnaev at his home in Cambridge after being tipped off by Russian intelligence, which suspected him of having "radical Islamic" sympathies. Tamerlan denied this to the agents. He said that he had "respect for all religions," visited a local mosque for prayer each Friday, and had few Muslim friends.

But Tamerlan also told the agents that he had been visited earlier by four mysterious, well-dressed young men claiming to be FBI agents. They spoke without any discernible accent, Tamerlan said, and promised to return after asking a few questions. Tamerlan said he never saw the four again, adding that he didn't know why anyone would be upset with him. The real FBI agents appeared to have accepted that explanation at face value.

Tamerlan would later be killed in the Watertown shootout following the marathon bombings. His younger brother, Dzhokhar Anzorovich "Jahar" Tsarnaev, now awaits the federal death penalty for his role. The FBI has offered no explanation for the four mysterious visitors who claimed to be their agents.