
In the April attack against Khan Sheikhoun, sarin gas was suspected agent that led to the deaths of at least 74 people.
Spicer's statement is unique -- the US has never warned of such an incident in Syria before it happened -- and potentially critically important as it could be a harbinger of a coming attack, or a justification for further US airstrikes in Syria.
The Syrian government has used sarin gas on multiple occasions, including two mass-casualty events: April's incident, and the 2013 sarin attack in the Ghouta suburbs of Damascus which likely killed more than 1400 people. Beyond this, it has used chlorine gas on countless occasions. Most of Syria's dead, however, have been killed by conventional weapons, and most of those who have died in Syria have died at the hands of the Syrian government and its allies.
Read more on the Ghouta attack here:

Stop Saying Only 355 Died in Chemical Weapons Attack
How many people died in the August 21st chemical weapons attack? In their official report, the United States government has said that they've documented at least 1,429 deaths, of which at least 426 are children. How do we know this is accurate?
View full page →A US official told CBS that an increase in underground activity was detected at the Shayrat airfield in Homs province, the same facility that the US says is responsible for the April attack. According to CBS, the unnamed official said that US airstrikes against that facility that followed that chemical attack spared the bunker due to concerns that the toxic material stored there could spread.
According to that report, the US has also picked up increased communications by the Syrian unit responsible for the chemical weapons, though CBS adds that according to their source it is not clear if that communication was due to preparations for another attack or something else like an inspection.

White House releases ominous statement about another possible Syria chemical attack
WASHINGTON -- The White House says it has found "potential" evidence that Syria is preparing for another chemical weapons attack. Press secretary Sean Spicer issued an ominous statement Monday evening that says the U.S. "has identified potential preparations" for another chemical attack by the Assad government that it says "would likely result in the mass murder of civilians, including innocent children."
View full page →The warning to Syria reportedly took place during a call between Russian Foreign Minister Sergei Lavrov and US Secretary of State Rex Tillerson. CBS reports that Lavrov underscored that Russia, too, wants to prevent chemical attacks, but classified the US statement and recent US airstrikes on Iran-backed militia forces in Syria and the shooting down of a Syrian military jet to be aggressive moves.
The Russian government also condemned last night's statement by the White House as "unacceptable." Reuters reports:
"I am not aware of any information about a threat that chemical weapons can be used," Kremlin spokesman Dmitry Peskov told a conference call with reporters on Tuesday.
"Certainly, we consider such threats to the legitimate leadership of the Syrian Arab Republic unacceptable."
We will provide rolling analysis throughout the day.
-- James Miller