According to this story here from sky news - declassified CIA files/interviews with former agents outline the US' involvement in Saddam Hussein's chemical attacks against Iran. Seems the US passed military intelligence to Iraq which they used in planning/carrying out chemical attacks, and then hid the evidence when Iran were trying to prove this to the UN.
It really just goes to show you can never be 100% sure of who is on what side, and how far they'll go in the name of self interest. Pretty ironic given the current situation in Syria. I really do fear what will happen next in Syria.