The U.S. Attorney's office alone does not have the power to agree that the cooperator will not be deported. See 28 C.F.R. 0.197 ("The Immigration and Naturalization Service shall not be bound, in the exercise of its authority under the immigration laws, through plea agreements, cooperation agreements, or other agreements with or for the benefit of alien defendants, witnesses, or informants, or other aliens cooperating with the United States Government, except by the authorization of the Commissioner of the Service or the Commissioners delegate. Both the agreement itself and the necessary authorization must be in writing to be effective, and the authorization shall be attached to the agreement."). Prior case law held that an Assistant U.S. Attorney could bind the INS. See, e.g., Margalli-Olvera v. INS, 43 F.3d 345, 353-54 (8th Cir. 1994); Thomas v. INS, 35 F.3d 1332, 1337-43 (9th Cir. 1994). After the regulation was promulgated, however, this is no longer true. A state law enforcement agency must file a Form I-854, endorsed by highest state law enforcement officer and the U.S. Attorney in the jurisdiction, with the approval of the Assistant Attorney General, Criminal Division, USDOJ, Washington, D.C., who certifies the application and forwards it to ICE if the witness is in the United States, or to the Secretary of State if the witness is outside of the country. Counsel may notify the local DRO that an S visa is being requested, and perhaps they will stop removal. See 8 C.F.R. 214.2(t)(4).

Thanks to Tova Indritz.