In a landmark discovery, a Turkish historian says he has found in a copy of a Jerusalem archive collection a “smoking gun” proving beyond doubt that the Ottoman Empire carried out the premeditated genocide of hundreds of thousands of Armenians and attempted to cover up evidence of the event.

After years of searching for irrefutable proof of the genocide, Taner Akcam, a Turkish-born scholar at Clark University in Worcester, Massachusetts, says he has uncovered a long-lost telegram used as evidence in military tribunals that convicted the planners of the mass killings, The New York Times reported Saturday.

“Until recently, the smoking gun was missing,” Akcam, who is considered one of the world’s leading authorities on the Armenian cause, told the paper. “This is the smoking gun.”

The telegram, originally written in secret code, is a request from a high-level Turkish official, Behaeddin Shakir, for details from the field about the deportations and killings of Armenians in the eastern Turkish region of Anatolia, the report said.

The document, found by Akcam in a copy of an archive held by the Armenian Patriarchate of Jerusalem, is a deciphered version of the original telegram that was used to help convict Shakir of planning the murders. According to Akcam, the discovery proves both the existence of the tribunals and, for the first time, the deliberate and willful official planning involved in carrying out the massacres.

Akcam said the find was “an earthquake in our field,” and expressed hope it would remove “the last brick in the denialist wall.”

The killings of up to 1.5 million Armenians by Ottoman Turks around the time of World War I is widely viewed by scholars as the first genocide of the 20th century.

Turkey, however, denies that the deaths constituted genocide, saying the toll has been inflated and that those killed were victims of war and civil unrest. Ankara has admitted that large-scale massacres took place, but says they were perpetrated in self-defense against what it describes as a Russian-inspired uprising by Armenians.

Monday marks the annual commemoration of the day when some 250 Armenian intellectuals were rounded up in what is regarded as the first step of the massacres.

For years, Akcam, along with other historians, has been searching for documents from the 1919-20 military tribunals to constitute firsthand proof of the genocide and subsequent coverup. With the court transcripts and original documents destroyed, researchers have thus far relied on summaries from Ottoman newspapers for information from the trials.

The telegram was discovered in a collection of court records, shipped out of Turkey in 1922 by Armenian leaders fearing they would be destroyed by Turkish nationalists who would later seize control of the country.

Brought to Jerusalem in the 1930s, the collection was put in an archive in the Armenian Patriarchate but was inaccessible to researchers, the report said. Recently, however, Akcam discovered that the entire collection had been photographed in the 1940s by an Armenian monk who passed the photos on to a nephew currently living in New York.

After hours of painstaking work sifting through the photos, Akcam was eventually able to match some of documents to the the coded letterhead of the Turkish Interior Ministry at the time.