A United Nations investigation has gathered testimony that Syrian rebels have been using sarin gas on their countrymen.

Chemical weapon use has been suspected in the Syrian civil war since at least 2012, and U.S. intelligence agencies thought the evidence pointed toward forces loyal to President Bashar al-Assad. But Carla Del Ponte, a member of a U.N. independent commission of inquiry on war crimes in Syria, said in an interview with a Swiss-Italian TV station that investigators have not yet seen concrete evidence that the government has used chemical weapons, according to Reuters. However, interviews with victims and doctors in the field point toward the nerve agent’s use by the rebelling forces, which rose up during the Arab Spring protests of 2011, Del Ponte said.

Sarin is a colorless, odorless substance that can be absorbed through the mouth, nose or skin and is estimated to be 500 times as toxic as cyanide. It attacks the nervous system through a mechanism similar to insecticides.

The substance was developed by German scientists shortly before the breakout of World War II. During the Cold War, both NATO and USSR-aligned countries made stockpiles of sarin. Two of the more notable incidents involving sarin gas use were the 1988 massacre of around 5,000 Kurdish civilians by Iraqi government forces during the Iran-Iraq war and the 1995 attack on the Tokyo subway by the Japanese cult Aum Shinrikyo, which killed 13.

The 130 countries that signed the Chemical Weapons Convention, finalized in 1993, agreed to destroy their existing stocks of chemical weapons and to not produce, stockpile or use any additional chemical agents. Syria is one of six nations that did not sign the treaty.

But what exactly does sarin gas do to the body?

Nerve agents like sarin belong to a class of compounds called organophosphates, which have a phosphorus atom at their core. They affect the body by preventing a victim’s nerves from breaking down a molecule called acetylcholine, which has a lot of different functions in the nervous system and throughout the body. This creates a buildup of acetylcholine, which overloads the muscles’ ability to respond to nerve signals. The muscles become weak and unresponsive, then seize up in a condition called flaccid paralysis. Other effects of excess acetylcholine include vomiting, respiratory failure, sweating and salivation.

Treating nerve agent victims requires quick action. One emergency management plan published in the Journal of Pharmacy Practice in November 2011 recommends that medical personnel remove contaminated clothing and wash victims with water to prevent further exposure. Doctors that see patients exposed to sarin gas will usually administer intravenous drugs like atropine to block the muscle effects and pralidoxime, which reinvigorates the body’s ability to break down acetylcholine.

Even with medical attention, the effects of nerve gas poisoning can linger. In 2005, Japanese researchers found that a significant proportion of the victims of the Tokyo sarin gas attack still had a range of symptoms a year after the attacks: eye problems, fatigue, headaches and fear.

If sarin gas was used by either side in the Syrian civil war, it’s likely that the substance was made relatively recently. Depending on the purity of the chemicals used to make sarin, it has a shelf life of somewhere between a few weeks and a few months.