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“If they had known he was there he might not have come back,” said Mr. Cohen.

He noted that two extremists with whom Tamerlan had associated while in Dagestan — including William Plotnikov, a Russian-born Canadian who was, like Tamerlan, an amateur boxer — had been killed in what Russian authorities referred to as shootouts shortly before the elder brother returned to the U.S.

“They killed those two; it’s possible they would have killed him too,” Mr. Cohen said. “They sure don’t have the same kind of due process there.”

The European Council on Foreign relations calls the human rights situation in the North Caucasus “dire,” adding that “extra-judicial killings, torture, and abductions continued to be common” last year.

Human rights activists say that security forces often employ “shoot-to-kill” policies, and the council notes that “murdered civilians were often presented as insurgents” by authorities.

“It’s hard to know what to believe,” Mr. Cohen said. “Intelligence agencies aren’t in the truth business.”