Updated 10:00 p.m.

The Syrian military used an exotic chemical weapon on rebels during an attack in the city of Homs, some U.S. diplomats now believe.

That conclusion — first reported by Foreign Policy’s Josh Rogin and laid out in a secret cable from the U.S. consul general in Istanbul — contradicts preliminary estimates made by American officials in the hours after the December 23 strike. But after interviews with Syrian activists, doctors, and defectors, American diplomats in Turkey have apparently rendered a different verdict. It’s important to note, however, that this was the conclusion of a single consulate within the State Department, and there is still wide disagreement within the U.S. government over whether the Homs attack should be characterized as a chemical weapons incident.

“We can’t definitely say 100 percent, but Syrian contacts made a compelling case that Agent 15 was used in Homs on Dec. 23,” an unnamed U.S. official tells Rogin.

Agent 15 is similar to 3-quinuclidinyl benzilate or BZ, a powerful hallucinogen that the American military tested out on its own soldiers during the Cold War. Its emergence on the Syrian battlefield would be nothing short of bizarre. While Syria is well-known to have a massive supply of chemical weapons, international observers haven’t ordinarily included BZ on that list.

More: Syria Dropped Hallucinogen Weapon on Rebels, Secret Cable Says