The execution took place in Raqqa, a city in eastern Syria that is the Islamic State’s self-declared capital, according to two prominent groups that monitor the Syrian civil war.

One of the groups – the Syrian Observatory for Human Rights – said in a report Friday that the 20-year-old man murdered his mother, who was in her 40s, “in front of hundreds of people near the post office building … where she was employed.”

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The Britain-based Observatory said the incident started with an argument between the mother and son over his commitment to the Islamic State. The mother objected to his membership in the group, prompting him to order her arrest.

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“His mother spoke with him and asked him to leave ISIS and leave Raqqa to go to a different area of Syria and Turkey,” Rami Abdulrahman of the Syrian Observatory said in a telephone interview, using an acronym for the Islamic State.

“… After that he told ISIS and, 1-2-3, they arrest his mother.” Although the Islamic State is prolific in publicizing its executions, Abdulrahman said he did not know whether the murder had been filmed.

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However, Mohammed al-Saleh, an activist from the Raqqa is Being Slaughtered Silently group, which also monitors the Syrian conflict, said the reason behind the mother’s arrest is still unclear.

She was accused of apostasy, he said, speculating that she could have been murdered because she was a member of a Syria’s Alawite religious minority. Alawites, whose members dominate the upper echelons of Syria’s government and military, are an offshoot of Shiite Islam and condemned as apostates by hard-line Sunni groups such as the Islamic State.

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“This woman is Alawite, yet she somehow managed not to have been arrested or killed for so long while living under Islamic State rule,” said Saleh, a nom de guerre. He declined to give his real name because of threats against his group by the Islamic State, which controls vast territory in parts of Syria and Iraq.

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He noted that the son, a resident of Raqqa, has a Sunni father and was known in the city for heavy drinking and public brawling before the Islamic State took over in early 2014.

“When the Islamic State came in, he was the first to join the group,” said Saleh, speaking by Skype. “It’s clear that this guy is mentally disturbed.”

He added that the son fired a pistol into the back of his mother’s head.

Abdulrahman of the Observatory said that since June 2014, his organization has documented the Islamic State’s execution of 3,707 people in Syria – a total that includes 2,001 civilians, 106 women and 77 children.