Activists say a member of ISIS executed his own mother in front of hundreds of people in Syria after she was accused of being an apostate, or someone who renounces the terrorist group's brand of Islam.

The militant, whom the activists identified as 21-year-old Ali Saqr, reportedly shot and killed his mother, Leena al-Qasem.

The public execution occurred outside a post office in Raqqa, ISIS' de facto capital in Syria, according to the activist groups Raqqa is Being Slaughtered Silently and the Syrian Observatory of Human Rights.

Qasem had reportedly been an employee at the post office near where she was executed, according to a member of Raqqa is Being Slaughtered Silently.

The New York Times called the killing "the latest in a chain of brutal and bizarre killings" that ISIS, which is also known as the Islamic State, ISIL, or Daesh, "uses, and often widely publicizes, in efforts to tamp down dissent and to attract recruits."

The Times reported that Saqr's mother was trying to persuade him to leave ISIS, but Raqqa is Being Slaughtered Silently said that was not the reason for the killing.

Raqqa is Being Slaughtered Silently tweeted out these photos that they say show Saqr:

#Raqqa the main reason that Ali Saker killed his mother he Accused her for Apostasy #Syria #ISIL #ISIS pic.twitter.com/4L1njaBROg — الرقة تذبح بصمت (@Raqqa_SL) January 8, 2016

This type of killing isn't inconsistent with ISIS ideology.

Michael Weiss and Hassan Hassan interviewed an ISIS fighter for the book "ISIS: Inside the Army of Terror" and asked what he would do if his father fought for a rival jihadist group in Syria. The fighter didn't hesitate:

Asked what he would do if his father were a member of Jabhat al-Nusra and the two met in battle, Abdelaziz replied promptly: 'I would kill him. Abu Obeida [one of the companions of the Prophet] killed his father in battle. Anyone who extends his hand to harm al-Dawla will have his hand chopped off.' Abdelaziz also called his relatives in the Bahraini army or security forces 'apostates' because his adoptive country's military was by then involved in a multinational coalition bombing campaign against ISIS led by the United States.

And ISIS has reportedly ordered fighters to kill family members before — The Times noted that last year, a Lebanese man went to Raqqa to try to persuade his son, who had become an ISIS fighter, to leave with others the son had convinced to go there. Family members said the father was then detained and killed.