Click to expand Image A civil defense member breathes through an oxygen mask, after what rescue workers described as a suspected chemical attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. © 2017 Reuters

Evidence keeps mounting that Syrian government forces were behind a chemical attack that killed nearly 100 people in Khan Sheikhoun in northern Syria in April. Today, a United Nations-appointed commission of inquiry released a report in which it concluded that “the Syrian air force used sarin in Khan Shaykhun, Idlib, killing dozens, the majority of whom were women and children.” The commission based its findings on interviews with 43 witnesses, satellite imagery, photographs, and videos, and says it has evidence the attack was conducted by a Sukhoi SU-22 aircraft, a type that only Syrian government forces use.

This is consistent with findings from other investigations. The Organisation for the Prohibition of Chemical Weapons (OPCW) found that victims in the Khan Sheikhoun attack were exposed to sarin, a deadly nerve agent, and a Human Rights Watch investigation found that all available evidence strongly suggests that Syrian government forces were responsible for the attack.

The Khan Sheikhoun attack was horrific, but it’s not the only time Syrian forces have used chemical weapons this year; the commission’s report details three similar attacks between March and July. Human Rights Watch concluded in May that the government’s use of chemical weapons has become widespread and systematic, and may amount to crimes against humanity.

However, so far those responsible for chemical attacks in Syria have faced no real consequences. After a joint UN-OPCW probe last year found that Syria had repeatedly used chemical weapons, Russia, together with China, vetoed a UN Security Council resolution imposing sanctions. The OPCW also responded to the findings by merely condemning the attacks and imposing more stringent inspections.

More evidence is likely to surface, as the UN-OPCW Joint Investigative Mechanism is also investigating the Khan Sheikhoun attack. As evidence of Syrian responsibility for chemical attacks mounts, both the UN Security Council and the OPCW should increase pressure on the Syrian government, and do more to bring those responsible to justice.

Syria’s repeated chemical attacks are a major challenge to the ban on chemical weapons, one of the strongest weapon prohibitions in international law. Unless there are real consequences for those who use chemical weapons, the perpetrators will likely view the world’s inaction as a green light to continue using them.