Just possessing it is enough. Just looking at it is enough. They can tell where you've been on your computer no matter how much you "scrub". If I came across something like this accidentally, I'd inform the FBI right away that these sites had inappropriate material regarding children.
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While I agree with lawyer Sargeant, I do have one extra comment.
I suggest you speak to an attorney before contacting the FBI. I always suggest to potential clients that they should never enter the legal arena without a lawyer by their side. You never know what unintended consequences you could run in to.
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