The following excerpt is from Bacon v. United States, 449 F.2d 933 (9th Cir. 1971):
Rule 46(b) and 3149 provide specific criteria for probable cause. Cf. Giordenello v. United States, 1958, 357 U.S. 480, 485, 78 S.Ct. 1245, 2 L.Ed.2d 1503. Before a material witness arrest warrant may issue, the judicial officer must have probable cause to believe (1) "that the testimony of a person is material" and (2) "that it may become impracticable to secure his presence by subpoena." These requirements are reasonable, and if they are met, an arrest warrant may issue.
The above passage should not be considered legal advice. Reliable answers to complex legal questions require comprehensive research memos. To learn more visit www.alexi.com.