During the flight to Ingushetia from Moscow, Mr. Yevloyev and Mr. Zyazikov had found themselves in close quarters for the first time in years. A few weeks later, Mr. Zyazikov told a reporter from Ren TV, a Russian television channel, that he did not even know that Mr. Yevloyev was on the plane with him that day and had no idea who killed him. But Yakhya’s lawyers said their history raised the question of whether the men had a confrontation, and whether the president made the call that set the detention in motion.

Yakhya’s team also had a stroke of luck: a police investigator came forward to say he had been ordered to falsify testimony. Jambulat K. Shankhoyev had authorized police officers to bring Magomed in for questioning that day  but, Mr. Shankhoyev acknowledged, he later discovered that he had been asked to do so after Mr. Yevloyev was already in the police car. “I understood I had been set up,” the investigator wrote in a statement to Ingushetia’s president and prosecutor. When the investigator confronted his superiors about this, he wrote, he was told to keep quiet.

Nevertheless, investigators rejected motions filed by Yakhya’s lawyers one after another. They offered circular logic: If the preliminary investigation pointed to an accident, what legal basis was there for gathering evidence for a murder case?

So there would be no deposition of Mr. Zyazikov, or of passengers on the airplane who might have seen the two men interact, or of Mr. Shankhoyev, the investigator charging a cover-up. Police phone records would not be subpoenaed to trace the officers’ conversations with officials before and after the killing. Yakhya’s lawyers would not be allowed to be present during a crime scene re-enactment, leaving them powerless to point out its weaknesses.

Yuri N. Turygin, the regional prosecutor in Ingushetia, said he prayed Magomed Yevloyev would survive the gunshot wound, aware of the turmoil that would result if he died. Yet he suggested that Mr. Yevloyev, with his history of defiance, probably provoked his captors as he was being driven to police headquarters, knowing that some of his supporters were in pursuit.

“In my view, what caused his behavior was his character,” Mr. Turygin said in an interview. “He is a former prosecutor, he enjoyed some authority, and that dictated his behavior. It was probably within the framework of the law, but it was on the edge of an insult. He could humiliate a police officer  he could say, ‘They are running after you, and when they catch up with you, they will show you.’ ”

It was Mr. Turygin’s office that initially opened an investigation into negligent homicide. The case was taken out of his hands a day later, when it was transferred to the federal investigative committee, based in Moscow. In any case, Mr. Turygin said that if Yakhya Yevloyev had a compelling argument that a murder charge should have been pursued, the judges had leeway to send the case back to the prosecutor.