Fentanyl is a synthetic narcotic and is primarily used in hospitals for post op pain control and intraop obtundation. It is short acting and measured in micrograms. This is where the problem occurs, newbies will screw up the dosing and inadvertently give thousands of mics more than intended. I imagine some guy in a back room lab mixing up a batch of coke and trying some of this and that. If pros can fuck up the doses some untrained guy easily overdo it. They probably stopped breathing, lost their airways and faded out.