Of course, local news sources relay no useful information. The typical Government Media Complex response came and went without a whiff. Chemical Cleanup. It always comes down to a Chemical Cleanup. After all, who would question a bunch of guys in NBC suits, in front of checkpoints? After all, they're there to protect you! You needn't question the men in orange rubber suits! If they need that type of protection, after all, then whatever's in there would be bad, nasty and horrible enough to kill you, so you right-mindedly stay away. Besides, they're armed and helicopters patrol constantly, playing spotlights against the low rolling hills, causing the sagebrush to rake wicked and terrifying shadows across the playa.
Your curiosity is of course, piqued. You want to run in, past the checkpoint guards, past the helicopters and their searchlights. You want to see what's back beyond the flashing yellow lights. You want to see it for yourself, you want to believe your own eyes. But you do not. The men in the orange rubber suits with their rifles rightly scare you off. It's not their weapons, oh no. It's the fact that they represent some sort of chemical agent creeping through the pinons in invisible wafts. What if it's Plague? What if it's nerve gas? What if it's aerosolized anthrax (you know, back after they toured with Public Enemy and therefore stopped being awesome)?
You don't know. You stay away. Frightened for your very life at what the heck is actually out there in the dark beyond the spotlights and concertina wire and men with rifles. You go on with your life, completely unaware that you were almost an eyewitness to an event, the likes of which could likely change the very course of world events.
In this scenario, we can assume only one occurance, one worthy of such a hackneyed but unquestionable cover-up methodology:
Flying Saucer Crash.