This question is both historical and chemistry-related, but I'm posting it here because I am more interested in the chemical part:
In 1997, Israel tried to assassinate Khaled Mashal, a high-rank official of Hamas. This was to be done using an extremely potent poison that was supposed to be sprayed on him topically and kill him within a few days without a trace (a "quiet" murder).
I searched for the name of this poison, but I was only able to find out that it was a modified version of the drug Fentanyl (here, page 16). It was probably an internal formulation of the Mossad, but I was wondering if the formula was ever publicly released or if anyone knows how Fentanyl's structure was modified in order to make it so deadly upon contact.