It's crankery out of the right-wing epistemic closure, regarding a Democratic National Committee staffer murdered in an attempted robbery. It started with blogs and the self-reinforcing YouTube circle and has progressed now to Fox News, which has promulgated some
real amazing claims about said crankery from an "independent investigator" (actually a Fox News contributor) and who continued to do so even after that contributor retracted them and claimed that Fox misrepresented him.
Rich's family has denounced the cranks at length. (Surprise.)
The Wikipedia page is even-handed and probably gentler to the cranks than deserved: https://en.wikipedia.org/wiki/Murder_of_Seth_Rich