A U.S. Supreme Court case earlier this year, Rehberg v. Paulk, extended the immunity given to witnesses against liability for testimony given at trial to witnesses testifying before a grand jury.
Rehberg v. Paulk additionally held that there is no difference between a law enforcement officer and a civilian witness for purposes of this immunity. In practice this means that an individual cannot sue a law enforcement officer for their grand jury testimony even if the officer lied or the testimony was malicious.
Rehberg v. Paulk involved a 42 U.S.C.§ 1983 action against a district attorney's chief investigator. The investigator testified before a grand jury in three separate indictments against the petitioner, all of which were subsequently dismissed due to insufficient evidence. The petitioner then filed a § 1983 claim charging the investigator with presenting false testimony to a grand jury and conspiracy to present false testimony to a grand jury.