The suspicious attack that killed 26 people in northern Syria last week exposed the difficulty of determining whether the Syrian regime has resorted to using chemical weapons, as well as the lingering uncertainty over how President Barack Obama would respond if what he has called a "red line" is crossed.
Current and former U.S. officials acknowledged that confirming a small-scale chemical weapons attack poses technical challenges that have been compounded by limitations on the ability of U.S. spy agencies to gather reliable intelligence, let alone air or soil samples, inside Syria.
The two factors are why U.S. intelligence analysts are still working to determine whether the attack near Aleppo on March 19 involved the use of chemical compounds. The Syrian regime and rebels have accused each other of unleashing chemical weapons.