Does the government have a privilege to withhold from disclosing the identity of an informer?

The government enjoys a privilege to "withhold from disclosure the identity of persons who furnish information of violations of law to officers charged with enforcement of that law." Roviaro v. United States, 353 U.S. 53, 59 (1957). That privilege is not absolute, for "[w]here the disclosure of an informer's identity, or of the contents of his communication, is relevant and helpful to the defense of an accused, or is essential to a fair determination of a cause, the privilege must give way." Id. at 60-61.

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