The stigma surrounding homosexuality makes it difficult to document ISIS killings and identify victims, rights groups say. Families and friends refuse to talk about victims. Gays under ISIS rule are terrified to speak, and most who flee abroad go into hiding.
ISIS' own announcements are the main source of information, but the group often does not name the victims, perhaps in deference to their families, who could lash out in anger at having their names publicly linked to homosexuals.
"Such a barbaric show of murder leaves LGBT individuals in constant state of fear and would deprive them of a normal life that any human being is entitled to," Alizadeh said.
Widespread public hostility leaves the community even more vulnerable.
"They are violating God's laws and doing something that is forbidden in Islam, so this is a legitimate punishment," said Hajji Mohammed, a resident of the ISIS-held northern Iraqi city of Mosul. There the group has thrown men suspected of being gay off the Insurance Building, a landmark about 10 stories high.
By employing the grisly method, ISIS aims to show radicals that it is unflinchingly carrying out the most extreme strains in Islam - a sort of "ideological purity" the group boasts distinguishes it even from other militants. The punishment "will protect the Muslims from treading the same rotten course that the West has chosen to pursue," ISIS proclaimed in its online English-language magazine Dabiq.
The Quran tells the story of Lot and the destruction of Sodom - and sodomy in Arabic is known as "liwat," based on Lot's name.
Men having sex with each other should be punished, the Quran says, but it doesn't say how - and it adds that they should be left alone if they repent. The death penalty instead comes from the Hadith, or accounts of the sayings of the prophet Muhammad. The accounts differ on the method of killing, and some accounts give lesser penalties in some circumstances.
ISIS bases its punishment on one account in which Muhammad reportedly says gays "should be thrown from tremendous height then stoned."
Before ISIS, the method was rarely used, though other militants have targeted homosexuals for death. During their rule in Afghanistan in the 1990s, the Taliban had their own method: The victim would be put in a pit and a stone wall would be toppled on top of them.
Most moderate Muslim clerics ignore the death penalty provisions, even as they fiercely denounce homosexuality. Across the Arab world, homosexuals have been arrested and sentenced to prison on charges linked to "debauchery" - and sometimes lashed in Iran and Saudi Arabia.
Omar, the man who watched the killings in Palmyra, said he remains shaken.
It began when ISIS militants blared on loudspeakers for men to gather. Then a black van pulled up outside the Wael Hotel, and Mallah and Salamah were brought out.
The first to be thrown off was Mallah. He was tied to a chair so he couldn't resist, then pushed over the side.
He landed on his back, broken but still moving. A fighter shot him in the head.
Next was Salameh. He landed on his head and died immediately. Still, fighters stoned his body, Omar said.
The bodies were then hung up in Palmyra's Freedom Square for two days, each with a placard on his chest: "He received the punishment for practicing the crime of Lot's people."