So that’s how the U.S. knows it was the Syrian government who launched chemical weapons. U.S. spies intercepted “panicked” phone calls between the Syrian Ministry of Defense and a leader of the chemical-weapons unit in the hours after the attack that left hundreds dead last week, Foreign Policy reported on Tuesday. American intelligence analysts say the phone calls—combined with local doctors’ accounts and the videos from the area—have made them certain that chemical weapons were used. Still unclear is why the Syrian government launched the attack, and at least one U.N. official linked President Bashar al-Assad’s brother to the attack, not the president himself. Meanwhile, al Qaeda vowed a “volcano of revenge” against the Syrian regime.