"Behind the walls of federal prisons nationwide, chaplains have been quietly carrying out a systematic purge of religious books and materials that were once available to prisoners in chapel libraries."
This lead summarizes all the important points of the article in a concise but engaging way. It doesn't give a great deal of detail, but provides just enough information to give the reader an idea of what the story is about and to convince the reader to go on.
Who: Chaplains - fairly general since it's the federal government that mandated that this be done, but enough info to let the reader know that it's someone in a position of power that's taking the books away.
What: a systematic purge of religious books and materials that were once available to prisoners - like the who, this does not tell the whole story but gives the reader the main idea. The word "systematic" lets you know that there has been some kind of conspiracy to make this happen.
Where: In federal prisons nationwide, in chapel libraries - the word "federal" lets you know that the government is probably behind what's happening. The fact that it's nationwide gives it more importance, since what is happening is not an isolated case.
The why and the when were not included, but were not really necessary. The facts laid out in the lead were interesting and just general enough to make the reader want to know the details.