Historically, under Section 27 of the Judiciary Act of 1789 U.S. Marshals had the common law-based power to enlist any willing civilians as deputies for necessary assistance in the execution of their duties.[23] In the Old West this was known as forming a posse, although under the Posse Comitatus Act, they could not use military troops in uniform representing their unit or the military service for law enforcement duties.

Under what condition is deputizing uniformed military personnel authorized?

[USC02] 10 USC Ch. 13: INSURRECTION