::::The book is cited in the article, as well as being listed in the bibliography, and it is devoted to the event; so I assumed that it is an acceptable reference. The FOX News piece includes a section entitled "Even stranger ideas", in which UFOs are mentioned; should that really disqualify the piece? For the EPSL paper, it mentions the event twice; the two relevant portions are below. 
 :::::Intriguingly this Permo–Triassic crater is the exact site of the 1908 Tunguska event, in which ~2000 km2 of forest [80] was flattened by an explosion generally attributed to a bolide ‘meteoroid explosion’. 
 :::::This evidence suggests that observed kimberlites may have been gas-rich magmas that ascended following the fracture paths created by the escape of even more gas-rich precursor ‘gas explosions’. Any such vapor-dominated ‘magmas’ would be extremely diffcult to spot within the geologic record as they can only be seen by their indirect e¡ects on preserved xenoliths and wallrock – and by deconstructing a very recent event or directly viewing the ‘fireball’ of an event like the 1908 Tunguska cryptoexplosion (a fireball was seen by several 1908 Tunguska eyewitnesses, which was the basis for it being classed as a meteor event, and such fireballs are now routinely searched for by atomic bomb test monitoring satellites). 
 ::::The AFP piece mentions an article in New Scientist, which I did not know about until just now. The article is only partially available online, but would seem to constitute another secondary source.  
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