Does the admissibility of evidence of threats against a witness require a showing that the threats caused a witness fear?

The other case cited by defendants, United States v. Thomas (7th Cir. 1996) 86 F.3d 647, also did not hold that the admissibility of evidence of threats against a witness requires a showing that the threats caused a witness fear. Instead, the federal appellate court concluded that the district court in a federal drug prosecution had abused its discretion under the Federal Rules of Evidence by admitting evidence of third party and anonymous threats to witnesses to "'boost'" their credibility, not to explain specific behavior relevant to their credibility. (Id. at p. 654.) The appellate court stated the following principle: "[T]hreat evidence has extremely limited probative value towards credibility, unless the evidence bears directly on a specific credibility issue regarding the threatened witness. For example, . . . to explain a witness' inconsistent statements, delays in testifying, or even courtroom demeanor indicating intimidation." (Ibid.) The opinion does not indicate whether the trial court gave a limiting instruction regarding the threats evidence. The appellate court ultimately concluded that the admission of the challenged threats evidence was harmless in light of the strength of the evidence against the defendants and evidence of threats against witnesses made directly by one of the defendants. (Id. at p. 655.)