I don't know the facts here regarding the grenade incident but I am familiar with grenades as I've thrown more than a few in my time. I cannot think it was an " accident." They can be thrown, dropped, rolled, kicked, etc and not go off " accidentally". There is a safety pin with a ring that extends through the top of the grenade to hold the lever in place. You put your finger in the ring, pull out the pin and throw. When you throw, or turn loose of the lever and it pops off, the fuse is activated and you'd better be getting rid of it. Regardless of seeing some war hero type in the movies pulling them with their teeth,it's a little harder to pull the pin out than that.When the pin is pulled, you can still continue holding down the lever and it still won't go off. When you release the lever, that's when you better be getting rid of it and pretty fast.
It is possible that some nuts were playing with one by pulling out the pin, holding down the lever and then re-inserting the pin. If so and they accidentally dropped it, then it could be in a way, called an accident.
They certainly are not something you play around with if you expect to reach old age !
There are several types now and the ones we used in Vietnam looked different than the ones used in WW 2 and Korea, which were called
" pineapples " due to their shape and appearance. The ones we used were smooth and rounder and not quite so big but more powerful.
At any rate, any grenade anywhere in the hands of thugs is bad news.
Hope this will be just an isolated incident !!