Grenade inside jail

I don't know how one would be convinced that someone came in with a grenade and passed through the security net of an installation like jail. And question comes... why somebody wanted a deadly device like grenade inside the jail?

The only possible conclusion is, some of the prisoners went out on that evening on a mission and came back safely after completing the mission partially....and since they were in a hurry while falling back, unfortunately they could not get rid of all the unexploded grenades....they carried one back to their den.

This practice is not totally new in our politics. Everyone must remember that in 1990 the then government let some prisoners out to attack demonstrators. I don't think we, the general people, can do anything about it. Our fate has become hostage to the politicians.