For more than a century, Turkey has denied any role in organizing the killing of Armenians in what historians have long accepted as a genocide that started in 1915, as World War I spread across continents. The Turkish narrative of denial has hinged on the argument that the original documents from postwar military tribunals that convicted the genocide’s planners were nowhere to be found.

Now, Taner Akcam, a Turkish historian at Clark University in Worcester, Mass., who has studied the genocide for decades by piecing together documents from around the world to establish state complicity in the killings, says he has unearthed an original telegram from the trials, in an archive held by the Armenian Patriarchate of Jerusalem.

“Until recently, the smoking gun was missing,” Mr. Akcam said. “This is the smoking gun.” He called his find “an earthquake in our field,” and said he hoped it would remove “the last brick in the denialist wall.”

The story begins in 1915 in an office in the Turkish city of Erzurum, when a high-level official of the Ottoman Empire punched out a telegram in secret code to a colleague in the field, asking for details about the deportations and killings of Armenians in eastern Anatolia, the easternmost part of contemporary Turkey.