Story highlights The 100th anniversary of the start of the mass killings will be commemorated Friday

Turkey and others reject the use of the word "genocide"

Most estimates of the deaths fall between 600,000 and 1.5 million

(CNN) The mass killings of Armenians in the Ottoman Empire, which began 100 years ago Friday, is said by some scholars and others to have been the first genocide of the 20th century, even though the word "genocide" did not exist at the time.

The issue of whether to call the killings a genocide is emotional, both for Armenians, who are descended from those killed, and for Turks, the heirs to the Ottomans. For both groups, the question touches as much on national identity as on historical facts.

Some Armenians feel their nationhood cannot be fully recognized unless the truth of what happened to their forebears is acknowledged. Some Turks still view the Armenians as having been a threat to the Ottoman Empire in a time of war, and say many people of various ethnicities -- including Turks -- were killed in the chaos of war.

In addition, some Turkish leaders fear that acknowledgment of a genocide could lead to demands for huge reparations.

Eiffel Tower to go dark on April 24 in memory of #ArmenianGenocide victims. #Turkeyfailed pic.twitter.com/aIwSurojXv — Armenian Genocide (@Genocideof1915) April 21, 2015

So, what do we know about happened in those fateful days? Here are some answers:

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