Secretary of State John Kerry says there is clear evidence that Syria's embattled leader Bashar al-Assad used chemical weapons, including highly toxic Sarin gas, against his own people.
"Blood and hair samples that have come to us through an appropriate chain of custody from East Damascus, from first responders, it has tested positive for signatures of Sarin," Kerry said.
The debate on how to respond will now play out in Congress.
President Obama has said he will seek lawmakers' approval before acting.
This approval is no sure thing, with some members of Congress deeply skeptical of any action.
"I think the line in the sand should be that America gets involved when American interests are threatened. I don't see American interests involved on either side of this Syrian war," said Republican Senator Rand Paul of Kentucky.
Kerry says the administration will talk strategy with members of Congress over the next week, but a vote will likely have to wait.
The House and Senate are not scheduled to be back in session until September 9.
"This is a clear failure of leadership, and if you feel so strongly about it, and if he doesn't want to take the action himself, then he should call us back into session tomorrow," said Long Island Republican Rep. Pete King.
Still, others say the President is acting responsibly.
"He and the country and the world would be stronger if Congress was supportive of his activities, because this is not just a short term effort, this is a longer term effort," said Democratic Senator Jack Reed of Rhode Island.
Other members of Congress say the United States needs to go further than just limited air strikes.
"The best way to eliminate the threat of Bashar Assad's continued use of chemical weapons, and by the way we know he's used them a number of times before, would be the threat of his removal from power," said Republican Senator John McCain of Arizona.
Kerry said the President did not need to go to Congress for approval to take military action and retains the right to strike even with a no-vote.
Now that he has made that request, however, what happens next is up in the air.