The Syrian government did not use chemical weapons against residents of Homs in a December attack, a U.S. State Department investigation shows, but did apparently misuse a riot-control gas in the incident, according to senior U.S. officials.

The investigation stemmed from allegations inside Syria about the use of chemical weapons during an attack on the city of Homs on December 23. The officials said the State Department launched a probe from its consulate in Istanbul after doctors and activists reported dozens of victims suffering from nervous system, respiratory and gastrointestinal ailments after inhaling the gas.

Foreign Policy's "The Cable" blog reported Tuesday that a secret diplomatic cable provided a "compelling case" that President Bashar al-Assad's military used chemical weapons in the attack.

The United States was informed of the incident by representatives of a non-governmental organization working in Syria, who told the U.S. consulate in Turkey that they believed a chemical attack took place in Homs, according to a U.S. official. The NGO set up some interviews for the consulate, which then wrote a cable discussing the concerns. The U.S. official said the cable noted that the evidence was inconclusive that there was a chemical attack.

However, the concern triggered a more extensive investigation by the State Department, with intelligence personnel assessing online videos of the attack and pictures of the victims. Chemical weapons experts and doctors experienced in treating patients exposed to chemical weapons were also consulted, according to U.S. officials. And interviews were conducted with Syrian doctors and activists inside the country by a U.S. partner there.

The gas was determined to be a "riot control agent" that was not designed to produce lasting effects, but became more dangerous when it was released in dense areas and was not dispersed in the air quickly, the officials said.

"It is meant to be short term," one of the officials said. "But just like with tear gas, if you breathe in an entire canister, that can have a severe effect on your lungs and other organs."

"That doesn't make it a chemical weapon, however," the official said.

Dr. Abu al Fida, who treated about 30 of the approximately 100 people who were affected by the mysterious gas, told CNN the victims' symptoms depended on their proximity to the substance.

People who were further away from the source suffered labored breathing, disorientation, hallucination, nervousness and lack of limb control, al Fida said.

But those closer to the source of the gas had much more severe symptoms, including paralyses, seizures, muscle spasms and in some cases blindness, he said. Six people were killed by the gas, the doctor said.

Those affected responded well to atropine, a medication used to treat people exposed to the nerve gas sarin, al Fida said.

The senior U.S. officials said the symptoms in those who inhaled the gas were similar to those in people exposed to Agent 15, an incapacitating gas controlled by the Chemical Weapons Convention, although it was later determined not to be Agent 15.

The officials also said that while some Syrian doctors on the ground were convinced the gas was a chemical weapon, others were not.

A senior Turkish diplomat told CNN that Turkey also conducted its own investigation into the chemical weapons allegations, but found the claims to be unsubstantiated.

President Barack Obama has said the use of chemical weapons by al-Assad's government would cross a "red line" which would trigger a robust U.S. and international response.

"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized," Obama first said in August. "That would change my calculus."

National Security Council Spokesman Tommy Vietor said that reports about the use of chemical weapons by the regime have "not been consistent with what we believe to be true about the Syrian chemical weapons program."

"The president was very clear when he said that if the Assad regime makes the tragic mistake of using chemical weapons, or fails to meet its obligation to secure them, the regime will be held accountable," Vietor told CNN.

Officials tell CNN there are no plans for more robust specific action in light of the investigation. A Deputies Committee meeting of top administration officials is scheduled for Thursday, but officials say they do not expect any policy shifts.