Updated 1/17/13 12:28 p.m.

The State Department is publicly discounting claims made by its own diplomats about a chemical weapons attack in Syria.

On Tuesday, Foreign Policy detailed a secret and previously unknown cable from the U.S. consulate in Istanbul which came to the explosive conclusion that Syrian government forces dropped a hallucinogen known as "Agent 15" on rebels in the town of Homs on December 23.

But less than a day later, State Department spokesperson Victoria Nuland has denied the report, saying that the Foreign Policy story "did not accurately convey the anecdotal information that we had received from a third party regarding an alleged incident in Syria in December."

"At the time we looked into the allegations that were made and the information that we had received, and we found no credible evidence to corroborate or to confirm that chemical weapons were used," she added. That's a major deal, because the international community has repeatedly told the Assad Regime in Syria that the use of chemical weapons is beyond unacceptable. The White House issued a statement along similar lines.

U.S. officials contacted by Danger Room said the information in the cable originated from a contractor hired by the State Department to monitor opposition media coming from Syria. After the attack in Homs, rebel activists posted gut-wrenching videos to YouTube of gasping victims crying out in agony. In the clips, opposition figures claimed that they had been hit with a poison gas – maybe a nerve agent, maybe a hallucinogen.

American experts could find little in the videos that corroborated either chemical weapons story. (For one thing, hallucinogens and nerve agents have almost opposite symptoms and treatment regimes.) In the hours after the attack, U.S. officials expressed skepticism about the rebel claims. The bit about Agent 15 seemed particularly odd; while the U.S. military experimented on its own troops with a similar hallucinogen called BZ, there was yet to be a proven case of the the agent being used in anger on the battlefield. Nevertheless, according to CNN, the State Department did ask "a U.S. partner" to follow up, interviewing Syrian doctors and chemical weapons specialists.

CNN says that the gas was determined to be a "riot control agent" – a broad category of weapons that includes tear gas, pepper spray, and Agent 15. None of these agents are designed to be deadly. But they're also not designed to be inhaled in large quantities. "Just like with tear gas, if you breathe in an entire canister, that can have a severe effect on your lungs and other organs," one official tells CNN.

In other words, if a chemical was used in Homs, it wasn't a weapon of mass destruction.

It almost certainly wasn't a hallucinogen, either. CNN also interviewed a doctor who said he treated victims in Homs with atropine. If that's true, it rules out the use of a hallucinogen like Agent 15 or BZ. Both are anticholinergic agents, blocking the neurotransmitters in the parasympathetic nervous system. One would only enhance the effect of the other. CNN says experts concluded that the Homs attack "was later determined not to be Agent 15."

The issue of chemical weapons continues to loom over the conflict in Syria. The regime's stockpiles are enormous, and they've been shown the willingness to prepare at least some of the agents for a possible attack. But for now, the U.S. government appears to have decided that, whatever happened in Homs, it didn't cross the chemical "red line" that the President on down had pledged would trigger outside intervention into the civil war.

Update: Dr. James Ketchum, who oversaw the American military's hallucinogen weapon experiments (and wrote about them in his book Chemical Warfare Secrets Almost Forgotten), doesn't believe such agents were used in the Homs attack, either.

Ketchum watched the YouTube videos of the aftermath, and doesn't see the signs of a hallucinogen like Agent 15 or BZ in the victims' responses. It's "Definitely not BZ or related anticholinergic," he writes in an email to Danger Room.

Those recovering from the attack "are all coherent and respond to questions with good attention. Most seem to complain of tightness in the chest and some require oxygen, but none show confusion, abnormal movements or dry mouth. Their pupils are not easy to see. All show anxiety – instead of being drowsy, restless, or non-responsive due to stupor from BZ-like drug," he adds. "Hours must have passed to get them all in beds and under medical care. BZ would have them in delirium by that time and they would be performing phantom acts (e.g. invisible cigarette being smoked)."

Ketchum wonders if a nerve agent like sarin wasn't used in Homs – it can produce some delirium during its early stages, and the atropine would've worked as a treatment. But U.S. experts quickly ruled out sarin or some other hyper-lethal nerve agent, because the gas had a strong odor (which sarin usually does not) and the victims in Homs manage to inhale a lot of the gas without dying.

So what was that mystery gas in Homs? Raffi Khatchadourian of the New Yorker puts together the clues, and comes up with the most credible suspect yet.