I think it's safe to say that Lockheed and Boeing didn't conspire to blow up a rocket on federal property -- but damn if the idea of Russia infiltrating spetznaz into the US defense industry for sabotage aimed at ensuring their control over the Western world's launch systems isn't a seductive piece of fiction.
The whole thing is very unlikely, but if true I'd say the most probable actor would be a lone disgruntled employee or other nutbar with a gun. It would fall in alongside the epidemic of lone nut shootings over the past year only the victim this time was a rocket.
I'd guess the requisite kind of gun and ammo could easily be obtained from among all the military surplus and other serious kit available on the gun show circuit.
I know you mean that 'nobody died', which is great, but the loss of a 200 million dollar satellite, a 50 million dollar launch vehicle (both representing tens of thousands of person-hours of work), and several further person-hours of time wastage investigating this incident, and months of delays in future launches... all this could hardly be called 'victimless'.
