He broadened his point to argue that much of the chaos gripping the Middle East today was a result of mistrust between communities over historical wrongdoings that no one is willing to confront.
“The past is not the past in the Middle East,” he said. “This is the biggest obstacle to peace and stability in the Middle East.”
Eric D. Weitz, a history professor at the City College of New York and an expert on the Armenian genocide, called Mr. Akcam “the Sherlock Holmes of Armenian genocide.”
“He has piled clue upon clue upon clue,” Professor Weitz added.
Exactly where the telegram was all these years, and how Mr. Akcam found it, is a story in itself. With Turkish nationalists about to seize the country in 1922, the Armenian leadership in Istanbul shipped 24 boxes of court records to England for safekeeping.
The records were kept there by a bishop, then taken to France and, later, to Jerusalem. They have remained there since the 1930s, part of a huge archive that has mostly been inaccessible to scholars, for reasons that are not entirely clear. Mr. Akcam said he had tried for years to gain access to the archive, with no luck.
Instead, he found a photographic record of the Jerusalem archive in New York, held by the nephew of a Armenian monk, now dead, who was a survivor of the genocide.
While researching the genocide in Cairo in the 1940s, the monk, Krikor Guerguerian, met a former Ottoman judge who had presided over the postwar trials. The judge told him that many of the boxes of case files had wound up in Jerusalem, so Mr. Guerguerian went there and took pictures of everything.
The telegram was written under Ottoman letterhead and coded in Arabic lettering; four-digit numbers denoted words. When Mr. Akcam compared it with the known Ottoman Interior Ministry codes from the time, found in an official archive in Istanbul, he found a match, raising the likelihood that many other telegrams used in the postwar trials could one day be verified in the same way.
For historians, the court cases were one piece of a mountain of evidence that emerged over the years — including reports in several languages from diplomats, missionaries and journalists who witnessed the events as they happened — that established the historical fact of the killings and qualified them as a genocide.
Turkey has long resisted the word genocide, saying that the suffering of the Armenians had occurred during the chaos of a world war in which Turkish Muslims faced hardship, too.
Turkey also claimed that the Armenians were traitors, and had been planning to join with Russia, then an enemy of the Ottoman Empire.
That position is deeply entwined in Turkish culture — it is standard in school curriculums — and polling has shown that a majority of Turks share the government’s position.
“My approach is that as much proof as you put in front of denialists, denialists will remain denialists,” said Bedross Der Matossian, a historian at the University of Nebraska and the author of “Shattered Dreams of Revolution: From Liberty to Violence in the Late Ottoman Empire.”
The genocide is commemorated each year on April 24, the day in 1915 that a group of Armenian notables from Istanbul were rounded up and deported.
It was the start of the enormous killing operation, which involved forced marches into the Syrian desert, summary executions and rapes.
Two years ago, Pope Francis referred to the killings as a genocide and faced a storm of criticism from within Turkey. Many countries, including France, Germany and Greece, have recognized the genocide, each time provoking diplomatic showdowns with Turkey.
The United States has not referred to the episode as genocide, out of concerns for alienating Turkey, a NATO ally and a partner in fighting terrorism in the Middle East. Barack Obama used the term when he was a candidate for president, but he refrained from doing so while in office.
This year, dozens of congressional leaders have signed a letter urging President Trump to recognize the genocide.
But that is unlikely, especially after Mr. Trump recently congratulated Mr. Erdogan for winning expanded powers in a referendum that critics say was marred by fraud.
Mr. Shakir, the Ottoman official who wrote the incriminating telegram discovered by Mr. Akcam, had fled the country by the time the military tribunal convicted him and sentenced him to death in absentia.
A few years later, he was gunned down in the streets of Berlin by two Armenian assassins described in an article by The New York Times as “slim, undersized, swarthy men lurking in a doorway.”