"The court must be alert to the possibility of "jigsaw" identification. One piece of information may on its own seem innocuous, but when taken together with other information known to a particular ... actor, it may lead to the identification of an individual with greater or lesser confidence [...] although the court must be alive to the threat of jigsaw identification, it must also be astute not to allow the threat to justify a blanket prohibition on disclosure of any piece of the jigsaw."