A global chemical weapons watchdog said Wednesday it has confirmed the banned nerve agent sarin and chlorine were “very likely” used in an attack in northern Syria last year.

The Organization for the Prohibition of Chemical Weapons said in a new report that sarin was probably used south of the city of Ltamenah on March 24, 2017. The watchdog also concluded that chlorine was likely used as a weapon the next day at Ltamenah Hospital and in the surrounding area.

The organization said its conclusions were based on witness testimony and sample analyses.

The OPCW’s fact-finding team was created in 2014 in response to numerous allegations of chemical attacks in Syria with a mandate to “establish facts surrounding the allegations.” The team was not mandated to assign blame for the chemical attacks.