Unidentified assailants shot dead seven members of Syria's White Helmets rescue service early on August 12 during a raid on their base in northwestern Syria.
The seven volunteers were all killed by bullets to the head.
"The civil defense center in Sarmin was the target of an armed attack by unknown assailants in which seven volunteers were killed," the White Helmets said in a statement.
"Two minibuses, some white helmets and walkie-talkies were stolen."
It was not immediately clear if the raid had political or criminal motives. Sarmin is in Idlib Province, which has witnessed clashes recently between rival insurgent groups.
The British-based Syrian Observatory for Human Rights said that the fatalities were discovered when volunteers arrived to start a shift and found the bodies of their colleagues.
The White Helmets have been working to rescue civilians in rebel-held areas since 2013.
They work exclusively in rebel-held areas and receive donations from several Western governments, including Britain, Germany, the Netherlands, and the United States.
President Bashar al-Assad's government and his ally Russia accuse them of being tools of their international donors.
They are known for their daring rescues, filmed and circulated on social media.
In 2016 they were nominated for the Nobel Peace Prize.