Where lies the line between undercover investigation and incitement of crimes?

Pinkerton Detective Agency detective James McParland, seen here some time in the 1880s (Photo credit: Wikipedia)

And how much do you let your snitches get way with in order to maintain their intell?
http://personalliberty.com/2013/12/27/fbi-allows-informants-to-commit-thousands-of-crimes/

These have been law enforcement dilemmas at least since the days of the Molly Maguires.

http://www.amazon.com/Ned-McAdoo-Molly-Maguires-Educational/dp/1466248564/ref=sr_1_1?s=books&ie=UTF8&qid=1378230855&sr=1-1&keywords=ned+mcadoo+and+the+molly+maguires

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