The United States now believes the Assad regime has used small amounts of chemical weapons in Syria, administration officials said Thursday.
The White House confirmed that U.S. intelligence agencies "assess with varying degrees of confidence" the use of chemical weapons, in a letter to Sens. Carl Levin (D., Mich.) and John McCain (R., Ariz.).
"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specially the chemical agent sarin," the letter reads. "This assessment is based on physiological samples. Our standard of evidence must buildon these intelligence assessments as we seek to establish credible and corroborated facts. For example, the chain of custody is not clear, so we cannot confim how the exposure occurred and under what conditions."
Defense Secretary Chuck Hagel said Thursday that administration believes the Assad regime, rather than opposition fighters, is responsible for the weapons.
"I have been in contact with senior officials in Washington today and most recently the last couple of hours on this issue," Hagel told reporters in Abu Dhabi. "We cannot confirm the origin of these weapons, but we do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime."
Blood samples from multiple people have tested positive for sarin, a nerve agent, officials told Danger Room Thursday.
President Barack Obama has repeatedly isolated the use of chemical weapons as an action that would cross a "red line" required for U.S. military action.
The White House said Thursday "intelligence assessments alone are not sufficient" in the Congressional letter for administration "decision-making."
"Only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making," the White House director of legislative affairs, Miguel Rodriguez, writes in the letter.
Hagel told reporters the United States will move to "fully investigate" the assessments through allies and the United Nations.
"The president has made it clear that the use of chemical weapons or the transfer of such weapons to terrorist groups would be unacceptable," Hagel said. "The United States has an obligation to fully investigate – including with all key partners and allies, and through the United Nations – evidence of chemical weapons use in Syria."
More than 70,000 people have died in the Syrian civil war, which began more than two years ago.
Update (2:14 p.m.): A senior White House said Thursday it is not yet clear whether the administration's red line has been crossed during a conference call with reporters, Foreign Policy's Josh Rogin reports:
A senior White House official said on the conference call that the intelligence community's assessment was not enough to determine that President Obama's red line regarding U.S. intervention in Syria has been crossed.
"We are continuing to do further work to establish a strong, firm, evidentiary basis to determine whether or not the red line has been crossed," the official said. "If we make a determination that the red line has been crossed... what we will be doing is consulting with friends and allies as to the next steps forward." [...]
"It is crucial, given our own history with intelligence assessments, including intelligence assessments on weapons of mass destruction... that we are able to present evidence that is airtight," the official said. "It is absolutely the case that the president's red line is the use of chemical weapons or the transfer of chemical weapons to terrorist groups. Our standard of evidence has to build on these intelligence assessments. We want to continue to investigate."
The white House official also declined to say what options might be on the table.