We simply don't know what type of weapons they were testing back then, or even now. Technology used by the Military or whoever the powers that be are, are completely shrouded in mystery and protected at all costs.
I mean - sure they are conspiracy theories, but just think about "The Bell" supposedly tested by the Nazis; or even more relevantly, Area 51. I don't think we will ever know what goes on there. We don't even know if supposed UFO's are our own technology or alien. That's how secretive they are. Even "cloaking" technology - we only have tidbits or rumours....meanwhile, cloaked aircraft could be flying over our heads as we speak! As it is now, so it was then. And probably even more so as it was the era of the Cold War.
I'm not fully behind selling this idea of an electromagnetic weapon, but it could be a possibility.
Regarding the Native tribes murder theory, it could be a possibility too. As far as I know, there was no retribution to the Khanty for the murders of non-Natives in the 1930's was there?
I think the government would wish to preserve relations with Native tribes. It is such a remote area. I think they pretty much regard it as their land; their rules.
Kind of like that missionary who was murdered by locals on an island near India I think it was, maybe last year?? The authorities didn't do anything, mostly because he had been warned not to go there, but went anyway and ended up being murdered.
Covering up a possible murder by Natives in the Ural mountains I think, would be paramount to preserving relations with the tribes and preserving peace. If it was exposed, there would be chaos, perhaps serious fighting and mass murder - non-Natives would seek vengeance....it would just be a political nightmare. I would think they'd like to keep it quiet to prevent this violent reaction??