"In the last 24 hours, we have learned through samples that were provided to the United States that have now been tested from first responders in east Damascus and hair samples and blood samples have tested positive for signatures of Sarin," Kerry told NBC's "Meet The Press."
Sarin is a nerve toxin so deadly that just one drop can kill a grown man. The fatality typically comes from cardiac arrest or suffocation, as overstimulated muscles around the heart and lungs eventually seize and stop working altogether.
A preliminary U.S. government assessment determined that 1,429 people were killed in the attack on Aug. 21 in the Damascus suburb of East Ghouta.
Only a few people in history have dared to use sarin gas.
"Bashar al-Assad now joins the list of Adolph Hitler and Saddam Hussein have used these weapons in time of war," Kerry said. "This is of great consequence to Israel, to Jordan, to Turkey, to the region, and to all of us who care about enforcing the international norm with respect to chemical weapons."
A limited U.S. strike appeared imminent until Saturday, when Obama said he would first seek Congressional approval. The debate will start on Sept. 9.
"Use of chemical weapons is unacceptable," Kerry told NBC. "And we cannot stand by and allow that to happen and create an impunity for its use. That would be the end of the chemical weapons norm. ... Now why go to Congress? Because the United States of America is stronger when the Congress of the United States representing the people and the President of the United States are acting together. And the president wants that strength represented in this initiative. "
The president's decision has been met with criticism from the Syrian opposition and derision from Syrian state media.
"We can't understand how you can promise to help those who are being slaughtered every day in the hundreds, giving them false hope, then change your mind and say let's wait and see," the Syrian National Coalition, a key group of Syrian dissidents, said in a statement.
"Whether the Congress lights the red or green light for an aggression, and whether the prospects of war have been enhanced or faded, President Obama has announced yesterday, by prevaricating or hinting, the start of the historic American retreat," Syrian state newspaper Al-Thawra wrote.
Nevertheless, it appears that the White House is certain that Assad committed the atrocity and will act accordingly.
"We know that the regime ordered this attack, we know they prepared for it," Kerry told CNN's "State of the Union." "We know where the rockets came from. We know where they landed. We know the damage that was done afterwards. ... we know that the regime tried to cover up afterwards, so the case is really an overwhelming case."
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