The Obama administration pushed forward Sunday on a new path toward military action in Syria, urging Congress to support the president's call.
Tests found signatures of sarin gas in blood and hair samples collected from the Damascus site of an alleged chemical weapons attack, U.S. Secretary of State John Kerry said Sunday on CNN's State of the Union.
The samples were collected separately from a United Nations investigation into the August 21 attack and provide further proof that the Syrian regime attacked its own people, Kerry said.
With "each day that goes by, this case is even stronger," he said, arguing that the United States must act.
"If you don't do it, you send a message of impunity," Kerry said. Iran, North Korea, and Hezbollah "will look at the United States and say 'Nothing means anything' -- that's what's at stake here," he said.
Syria denies using chemical weapons on its people and blames the rebels.
Kerry called the evidence "overwhelming" Sunday, and the Arab League issued a statement blaming the Syrian government for the attack.
But the United Nations argued that world leaders should wait until U.N. investigators determine whether chemical weapons were used.
"The U.N. mission is uniquely capable of establishing in an impartial and credible manner the facts of any use of chemical weapons," Martin Nesirky, spokesman for Secretary-General Ban Ki-moon, said repeatedly at a news conference.
So when will the results be ready? The U.N. won't give a timeline, Nesirky said. "It's being done as fast as it is possible to do within the scientific constraints."
Samples will be delivered to laboratories Monday, he said.
The investigation involves a strict chain of custody and clear guidelines, he said, adding that two Syrian government officials monitored the process.
But even when it's done, the U.N. will only say whether chemical weapons were used -- not who was responsible.
Obama's last-minute Syria switch
U.S. military action appeared imminent until Saturday, when Obama announced he would first seek lawmakers' approval.
Obama made a last-minute decision Friday evening to seek congressional authorization before any military action, senior administration officials told reporters.
"While I believe I have the authority to carry out this military action without specific congressional authorization, I know that the country will be stronger if we take this course, and our actions will be even more effective," Obama said Saturday. The 1973 War Powers Act technically allows him to strike without such approval.
Lawmakers officially come back from recess on September 9.
But some members of Congress arrived on Capitol Hill Sunday for a classified briefing on Syria with White House, State Department and Pentagon officials.
Many of them said they remained skeptical and undecided about how to respond to the August 21 purported chemical weapons attack that U.S. officials have said killed more than 1,000 people in Syrian rebel strongholds.
"There was a great deal of skepticism in the room about the utility, effectiveness and support that we would have for the kind of strike that the president has proposed," said Rep. Jim Himes, D-Connecticut. "There's not a lot of skepticism, frankly, about whether or not this was an attack carried out by the Syrian regime. While nobody would say that it's been proven, the vast bulk of the evidence suggests that this was an attack carried out by the Assad regime."
Sen. John McCain told CBS' "Face the Nation" that Obama had invited him to a Monday meeting at the White House to discuss the next steps in Syria. McCain, who has been pushing for military intervention in Syria, said he had questions for the president.
"I want to find out whether there is a plan and a strategy. I want to find out whether this is just a pinprick that somehow Bashar Assad can trumpet that he defeated the United States of America," McCain told CNN. "But I will say that if Congress overrules a decision of the president of the United States on an issue of national security, that could set a catastrophic precedent in the future. It would be a very dangerous precedent to be setting."
Global debate surges over Syria
At a meeting in Cairo Sunday, Arab League foreign ministers condemned the chemical weapons attack, urging the international community to take action and calling for the prosecution of those responsible.