Mr. Emwazi apparently was set on the path to radicalization after being detained by the authorities on a visit to Tanzania in 2009 for a safari after graduation. He was accused by British intelligence officers of trying to make his way to Somalia.

Friends of Mr. Emwazi told The Washington Post that he and two other friends — a German convert to Islam named Omar and another man, Abu Talib — never made it to the safari. On landing in Dar es Salaam, the Tanzanian capital, in May 2009, they were detained by the police and held overnight and eventually deported, they said. Later, Mr. Emwazi said that an officer from MI5, Britain’s domestic security agency, had tried to recruit him.

Asim Qureshi, a research director at CAGE, a British advocacy organization opposed to the “war on terror,” civil rights abuses and erosion of due process, met with Mr. Emwazi in the fall of 2009. “Mohammed was quite incensed by his treatment, that he had been very unfairly treated,” Mr. Qureshi told The Post.

But in a statement on Thursday, Mr. Qureshi repeated that he could not identify Jihadi John as Mr. Emwazi with complete certainty. Mr. Qureshi said that two years of communications with Mr. Emwazi highlighted “interference by the U.K. security agencies as he sought to find redress within the system.”

Mr. Emwazi moved to Kuwait, his birthplace, shortly after his detention to work for a computer company, and he returned to London at least twice, Mr. Qureshi said. British counterterrorism officials detained Mr. Emwazi in June 2010, fingerprinting him and searching his belongings. In July of that year, Mr. Qureshi said, Mr. Emwazi was not allowed to return to Kuwait, which had apparently refused to renew his visa and blamed it on the British government.

“I had a job waiting for me and marriage to get started,” he wrote in a 2010 email to Mr. Qureshi. “But now I feel like a prisoner, only not in a cage, in London. A person imprisoned & controlled by security servicemen, stopping me from living my new life in my birthplace & my country, Kuwait.”