A panel of witnesses recounted Wednesday for a House committee how Syrian President Bashar al-Assad’s regime has used chemical weapons, one day after Secretary of State John Kerry said he was certain that Assad had used chlorine gas against his own people.

Mohamed Tennari, a medical director of an often-bombed field hospital in Syria’s Idlib province, recounted for the House Foreign Affairs Committee the first time he responded to a chlorine attack in March, and learning that his friend’s family had been saturated with the poisonous gas before they died in his hospital.

“One of the barrel bombs fell through a shaft in their home, filling the ventilation with chlorine as it broke open,” Tennari said. “Their basement became a makeshift gas chamber.”

The congressional testimony adds to the evidence that Assad continues to use chemical weapons, even after agreeing in September 2013 to turn over a cache after the United States and other nations threatened retaliation in response to a sarin gas attack a month prior that killed more than 1,500 people in the Damascus suburb of Ghouta. In April, Human Rights Watch released a report strongly suggesting that chlorine gas was used in six different barrel bomb attacks in Idlib province between March 16 and March 31.

The international Chemical Weapons Convention bans chlorine use in weapons, but the chemical itself was exempt from the 2013 deal because of its industrial uses, including in water purification.

Anne Sparrow, an assistant professor of global health at the Icahn School of Medicine at Mount Sinai, told the House committee Wednesday that the Assad regime’s use of chlorine gas is “emblematic of its war-crime strategy.” She has spent the past two years along the Syrian border helping train doctors.

“The Assad regime has transformed a principal element of public health into a tool of a disease and terror,” Sparrow said.

Sparrow and the other witnesses, including former ambassador to Syria Robert Ford, mostly supported the implementation of no-fly zone, though Ford emphasized that if a no-fly zone is to be put in place it must be negotiated through Turkey, Saudi Arabia, Qatar and Jordan to ensure that “such a military operation is used as a tool to help reach the larger national political negotiation that Syria so desperately needs.”

“A no-fly zone … could be used as leverage to get Assad to the negotiating table,” Ford added.

The Assad regime continuously denies the use of chlorine gas and  employment of the extremely lethal and rudimentary barrel bombs, but Samantha Power, U.S. ambassador to the United Nations, has put pressure on the organization to hold Assad accountable and make it clear to the world that chlorine needs to be taken seriously as a chemical weapon.

“At the Security Council, we’ve just in the last few months secured another resolution,” Power told the House Foreign Affairs Committee  this week. “To make it very clear to the world that just because chlorine is a household product doesn’t make it not a chemical weapon when it is put in a barrel bomb and dropped on civilians.”