Which poison was used in the failed attempt to assassinate Khaled Mashal of Hamas?
According to several sources online the chemical weapon used in the failed assissination of Mashal was levofentanyl.

This Wikipedia article states:


  On September 25, 1997, Mashal was injected in the ear with a toxin
  (thought to have been a derivative of the synthetic opiate Fentanyl
  called Levofentanyl).


Another source reads:


  In September 1997, an Israeli hit squad was sent to Amman to
  assassinate Khalid Mishal, the Hamas leader. The chosen instrument was
  levofentanyl, a deadly poison that leaves no traces and produces
  effects like a heart attack. It was administered by a slight physical
  touch.


And another:


  ...The agents entered the city using Canadian passports and waited for
  Mashal at his office. As he was entering, one agent injected a
  chemically modified toxin called levofentanyl into his left ear.


Concerning the structure I don't think it was ever publicised considering the case was high profile (perhaps all traces were taken offline or can be found in deep when-no one knows)

I've been trying to figure out how fentanyl could have been modified, but have no clue either.