By 2012, the old secularist guard had been quelled and the Gulenists and the A.K.P. were left more or less alone to run Turkey. It took less than two years before the two Islamist groups developed distrust and, ultimately, enmity. This tension came to a head in December 2013, when Gulenist police officers and prosecutors arrested dozens of government officials in a corruption investigation, most likely in the hope of toppling Mr. Erdogan, who condemned the inquiry as a “coup attempt.” At the time, this sounded like a self-serving exaggeration.

But the bloody plot of July 15 is far more destructive than anything Turkey has seen in recent years. Notably, the plot came as Mr. Erdogan was supposed to be planning a major purge of suspected Gulenists from the military. The military’s chief of staff, who opposed the coup, identified the rebellious officers as Gulenists. One plotter even reportedly confessed to acting under orders from the Gulen movement.

Given the Gulen community’s hierarchical structure, all of this makes Mr. Gulen a prime suspect. Of course, the truth can come out only in a fair trial. Unfortunately, Turkey is not good at those — especially given Mr. Erdogan’s control over the judiciary and the ferocious polarization in the country today. But the United States government can try to negotiate with its Turkish counterparts to extradite Mr. Gulen, as Turkey’s government is now requesting, on the condition of a fair trial.