The United States intercepted communications between the Syrian military and chemical weapons experts ahead of a sarin gas attack that left dozens of civilians dead last week, according to a report Wednesday evening.

According to CNN, a senior U.S. official said the military and intelligence community picked up on the discussion about the chemical weapons attack as part of a review effort to confirm whether Syrian President Bashar Assad was behind the attack amid the country’s six-year-plus civil war. The official emphasized that the U.S. was unaware of the chemical weapons plot before the attack happened.