Updated at 3:55 p.m. ET 
 
 As the Obama administration considers how to respond to the use of chemical weapons in Syria, Vice President Joe Biden said Tuesday that there is "no doubt" that Syrian President Bashar al Assad was responsible for the Aug. 21 chemical weapons attack near Damascus. 
 
 Speaking at the American Legion National convention in Houston, Texas, the vice president laid out the rationale behind that assertion. 
 
 "We know that the Syrian regime are the only ones who have the weapons," Biden said, "have used chemical weapons multiple times in the past, have the means of delivering those weapons, have been determined to wipe out exactly the places that were attacked by chemical weapons." 
 
 And instead of cooperating with international investigators, he continued, "the government has repeatedly shelled the sites of the attack and blocked the investigation for five days." 
 
 Biden said the administration's national security team has been in "close consultation" with their foreign counterparts. "Those who use chemical weapons against defenseless men, women and children should and must be held accountable," he said. 
 
 Questions about Assad's chemical weapon use "preposterous," W.H. says 
 
 In the White House press briefing earlier Tuesday, White House spokesman Jay Carney said the United States must respond to the use of chemical weapons in Syria because "allowing the use of chemical weapons on a significant scale to take place without a response would represent a significant challenge or threat to the United States' security interests." 
 
 While Carney said "there must be a response," he noted that President Obama has yet to decide what course of action he'll take. 
 
 "A decision about the use of military force has not been made," he said. "His options are many, and they include a variety of options that are not limited to the use of force." 
 
 The potential for a military strike in Syria has triggered some concern in on Capitol Hill, where lawmakers are pointing out that the president cannot legally use military force without congressional approval. As of early Tuesday afternoon, nearly two dozen lawmakers had signed onto a letter calling on the president to get Congress' authorization before acting militarily. 
 
 "Engaging our military in Syria when no direct threat to the United States exists and without prior congressional authorization would violate the separation of powers that is clearly delineated in the Constitution," the letter says. "If you deem that military action in Syria is necessary, Congress can reconvene at your request. We stand ready to come back into session, consider the facts before us, and share the burden of decisions made regarding U.S. involvement in the quickly escalating Syrian conflict." 
 
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 President Barack Obama said he had not yet made his decision regarding a U.S. strike on Syria during an interview with PBS NewsHour senior correspondents Judy Woodruff and Gwen Ifill. The president said that Syrian president Bashar al-Assad's regime alleged use of chemical weapons would factor into his calculation and he warned that the Assad should be held accountable. 
 
 Watch the PBS NewsHour full interview.
– John Kerry said it. Joe Biden said it. But now the person who truly matters has said it: President Obama has concluded that Syria used chemical weapons, he tells PBS NewsHour. Obama, however, said he has not made a decision about a possible military strike. The president said it's clear that chemical weapons were used, and he said the US has ruled out the possibility that the rebels were responsible, as Bashar al-Assad's government claims. "We have concluded that the Syrian government, in fact, carried these out, and if that's so, then there need to be international consequences." The US is in consultation with its allies, he said, adding that "I have no interest in any kind of open-ended conflict on Syria." Pressed on what such a limited action would accomplish, Obama said it would send a "pretty strong signal" to the Assad regime that it must stop the chemical attacks. The bigger message to Assad? "You are not only breaking international norms and standards of decency, but you're also creating a situation where US national interests are affected."