Defense Secretary Chuck Hagel said Thursday the U.S. intelligence community believes Syrian President Bashar Assad’s regime used the chemical weapon sarin in the country’s civil war.

The assessment was detailed in a letter to select members of Congress, which the administration has released.

Related: Dr. Manny explains sarin gas 

Sarin, according to the Centers for Disease Control and Prevention, is a “clear, colorless, and tasteless liquid that has no odor in its pure form,” and can evaporate and spread as a vapor into the environment.

Developed in 1938 in Germany as a pesticide, sarin has been used in the Iran-Iraq War of the 1980s and in two terrorist attacks in Japan in the 1990s, the CDC says.

People can be exposed to sarin through skin or eye contact, contamination of food and water, by breathing air, or through another person’s clothing – if they have been in contact with it.

When sarin enters a person’s body, it prevents the body’s “off switch” for glands and muscles from functioning properly, causing glands and muscles to constantly be stimulated, the CDC says.

The resulting health effects are difficulty breathing, fatigue, blurred vision, excessive sweating, confusion and varying levels of heart rate and blood pressure.

Exposure to a large dosage of sarin gas can result in paralysis and respiratory failure, leading to death.

Click for more information from the CDC.