A police official source in Makhachkala, Dagestan, told NBC News on Sunday that the Russian internal security service reached out to the FBI last November with some questions about Tamerlan, and handed over a copy of case file on him.

Tsarnaev had first popped up on the local police radar in Dagestan last summer, the source said. During routine surveillance of an individual known to be involved in the militant Islamic underground movement, the police witnessed Tamerlan meet the latter at a Salafi mosque in Makhachkala, the police official said.

It was one of six times in total that surveillance officials witnessed Tsarnaev meeting this militant at the same mosque, according to the police official.

The militant contact later disappeared, the police official said, but so did Tsarnaev before investigators had a chance to speak with him. The FBI never responded, according to the Dagestani police official.