What is the legal test for disclosing the identity of a confidential informant?

the identity of confidential informants." United States v. Fields, 113 F.3d 313, 324 (2d Cir. 1997). Disclosure obligations depend on "the particular circumstances of each case, taking into consideration the crime charged, the possible defenses, the possible significance of the informer's testimony, and other relevant factors." Roviaro v. United States, 353 U.S. 53, 62 (1957). In general, "[t]he defendant bears the burden of showing the need for disclosure of an informant's identity and to do so must establish that, absent such disclosure, he will be deprived of his right to a fair trial." Fields, 113 F.3d at 324 (internal citation omitted). "The defendant is generally able to establish a right to disclosure where the informant is a key witness or participant in the crime charged, someone whose testimony would be significant in determining guilt or innocence." United States v. Saa, 859 F.2d 1067, 1073 (2d Cir. 1988) (internal quotation marks omitted). "Speculation that disclosure of the informant's identity will be of assistance is not sufficient to meet the defendant's burden . . . ." Fields, 113 F.3d at 324. The decision whether to order disclosure lies within the sound discretion of the district court. Id.