Seven members of the controversial Syrian Civil Defense group, better known as the White Helmets were killed by “unknown armed men” in the group’s office in the town of Sarmin, Idlib province, on Saturday morning, the group said on Twitter.

According to the organization, this was “single most deadly attack” against its members. It was allegedly carried out by “a criminal gang,” it added.

“Two minibuses, some white helmets and walkie-talkies were stolen,” the White Helmets said, as cited by AFP.

An opposition activist told AP on condition of anonymity that the attack might have been carried out by an Islamic State cell in the town to show that Idlib province is not safe.

No group has claimed responsibility for the attack so far.