A boy affected by a gas attack is treated at a hospital near the Turkish border in Syria, Apr. 21, 2014. Reuters

United States officials believed that the nerve agent sarin was used in Tuesday’s chemical attack in Syria, an unidentified government source told Reuters. The deadly chemical was first developed by Nazi scientists in 1938 and was banned in 1993 under international law.

Tuesday’s attack in Khan Sheikhoun, described as among the worst in the country’s entire civil war, caused the death of at least 100 people, according to reports. Others were severely injured. Victims of the gas could be seen in videos and photos from the attack foaming at the mouth and struggling to breathe.

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“They were unconscious,” Mohammed Hassoun, a witness to the attack, told the Associated Press. “They had seizures and when oxygen was administered, they bled from the nose and mouth.”

Sarin is among the most deadly and fast-acting chemical agents, according to the Centers for Disease Control and Prevention. When inhaled or absorbed through the skin, the potent pesticide paralyzes lung muscles and parts of the nervous system, causing suffocation, convulsions, paralysis and death.

“Sarin is 26 times more deadly than cyanide gas,” the World Health Organization’s description said. “Just a pinprick-sized droplet will kill a human.”

U.S. government officials believed Syrian President Bashar Assad’s forces were responsible for the attack, according to Reuters. Assad joined the Chemical Weapons Convention in 2013, pledging to destroy his stockpile of chemical weapons. The Syrian military adamantly denied having a part in the attack.

“We deny completely the use of any chemical or toxic material in Khan Sheikhoun town today and the army has not used nor will use in any place or time neither in past or in future,” the Syrian army command said in a statement.

Sarin has been used in attacks in the past. The deadly nerve gas was pinpointed as the culprit for an attack by Syrian forces in a Damascus suburb in 2013, according to the AFP. U.S. officials blamed Assad for the attack, which killed more than 1,400 people.