The Obama administration has said repeatedly that the Assad regime's use of chemical weapons would be a "game changer" and would cross a "red line."
Hagel argued today that is it unclear whether red line has been crossed and said more evidence was needed.
"We need all the facts. We need all the information. What I've just given you is what our intelligence community has said they know," he said. "They are still assessing and they are still looking at what happened, who was responsible, and the other specifics that we'll need."
Hagel would not elaborate on evidence studied by the intelligence community and when pressed on what he meant by the community's varying degrees of confidence, he responded: "It means that we still have some uncertainties about what was used, what kind of chemicals were used, where it was used, who used it."
"Given the stakes involved, and what we have learned from our own recent experience, intelligence assessments alone are not sufficient," wrote the White House's liaison to Congress Miguel Rodriguez. "Only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making."
ABC's Luis Martinez at the Pentagon, Mary Bruce at the White House and Sunlen Miller on Capitol Hill contributed to this report.