I remember it from the 2002 Moscow Theater Hostage Crisis. The gas that was released into the theater to knock out the hostage takers was Fentanyl vapor, or that of a Fentanyl derivative. That was (I think) the first time I heard of it.
#3 I remember the gas killed most of the hostages!
How is Fentanyl made?
I would think following the ingredients being purchased would lead to those that make it. China is said to be a big manufacturer.