Can I promise a source confidentiality, even if I know the information was obtained illegally?

Yes, provided you are blameless in the illegal interception of the information. The U.S. Supreme Court held in Bartnicki v. Vopper that the First Amendment, in the interest of securing news and thereby fostering a robust and uninhibited debate on public issues, protects the media for publishing information of public concern that they know was obtained unlawfully by a source but where they did not participate in or encourage the illegal interception of the information.

According to the Court, the interest in deterring criminal conduct by third parties could not justify the suppression of speech by media parties caused by the law at issue, which punished the media for the mere disclosure of information contained in an illegally intercepted communication. The Court refused, however, to make any categorical decision about whether the media can ever be punished for the publication of truthful information, noting “there are some rare occasions” when the interest in deterring criminal conduct by third parties could justify a law that suppresses speech by other parties, including members of the media.

The Bartnicki Court also made clear that its holding is limited to the act of disclosing unlawfully obtained information and does not apply to the primary act of obtaining the relevant information unlawfully. Thus, neither journalists nor their sources are immune from prosecution for conduct that violates valid laws -- those criminalizing private wiretapping or the theft of documents, for example -- regardless of the newsworthiness of the information contained therein. Indeed, the federal government’s aggressive prosecution of former U.S. Army intelligence analyst Bradley Manning for the illegal act of turning over classified documents to an online publication is a compelling illustration of the Bartnicki limitation.