(1)    Rule of privilege. The federal government or a state or subdivision thereof has a privilege to refuse to disclose the identity of a person who has furnished information relating to or assisting in an investigation of a possible violation of law to a law enforcement officer or member of a legislative committee or its staff conducting an investigation.

   (2)   Who may claim. The privilege may be claimed by an appropriate representative of the federal government, regardless of whether the information was furnished to an officer of the government or of a state or subdivision thereof. The privilege may be claimed by an appropriate representative of a state or subdivision if the information was furnished to an officer thereof.

   (3)   Exceptions.

      (a)    Voluntary disclosure; informer a witness. No privilege exists under this rule if the identity of the informer or the informer’s interest in the subject matter of the informer’s communication has been disclosed to those who would have cause to resent the communication by a holder of the privilege or by the informer’s own action, or if the informer appears as a witness for the federal government or a state or subdivision thereof.

      (b)    Testimony on merits. If it appears from the evidence in the case or from other showing by a party that an informer may be able to give testimony necessary to a fair determination of the issue of guilt or innocence in a criminal case or of a material issue on the merits in a civil case to which the federal government or a state or subdivision thereof is a party, and the federal government or a state or subdivision thereof invokes the privilege, the judge shall give the federal government or a state or subdivision thereof an opportunity to show in camera facts relevant to determining whether the informer can, in fact, supply that testimony. The showing will ordinarily be in the form of affidavits but the judge may direct that testimony be taken if the judge finds that the matter cannot be resolved satisfactorily upon affidavit. If the judge finds that there is a reasonable probability that the informer can give the testimony, and the federal government or a state or subdivision thereof elects not to disclose the informer’s identity, the judge on motion of the defendant in a criminal case shall dismiss the charges to which the testimony would relate, and the judge may do so on the judge’s own motion. In civil cases, the judge may make an order that justice requires. Evidence submitted to the judge shall be sealed and preserved to be made available to the appellate court in the event of an appeal, and the contents shall not otherwise be revealed without consent of the federal government, state or subdivision thereof. All counsel and parties shall be permitted to be present at every stage of proceedings under this subdivision except a showing in camera at which no counsel or party shall be permitted to be present.

      (c)    Legality of obtaining evidence. If information from an informer is relied upon to establish the legality of the means by which evidence was obtained and the judge is not satisfied that the information was received from an informer reasonably believed to be reliable or credible, the judge may require the identity of the informer to be disclosed. The judge shall on request of the federal government, state or subdivision thereof, direct that the disclosure be made in camera. All counsel and parties concerned with the issue of legality shall be permitted to be present at every stage of proceedings under this subdivision except a disclosure in camera at which no counsel or party shall be permitted to be present. If disclosure of the identity of the informer is made in camera, the record thereof shall be sealed and preserved to be made available to the appellate court in the event of an appeal, and the contents shall not otherwise be revealed without consent of the appropriate federal government, state or subdivision thereof.