Authority of FBI Agents, Serving as Special Deputy United States Marshals, to Pursue Non-Federal Fugitives: Memorandum for Howard M. Shapiro, General Counsel, Federal Bureau of Investigation [February 21, 1995] [open pdf - 53KB]
"You have requested our opinion on the authority of FBI agents, serving as Special Deputy United States Marshals, to participate in federal-state task force efforts to locate and arrest fugitives charged with violations of state law where federal process is neither outstanding nor anticipated. Our conclusions on this matter may be summarized as follows: (1) Regardless of whether federal process is outstanding or anticipated, FBI agents have authority to investigate (and sometimes arrest) fugitive felons when there is a reasonable basis to believe that doing so will detect or prevent the commission of any federal crime, including violations of the Fugitive Felons Act [FFA]. That may include situations where a state fugitive has not yet crossed state lines but has engaged in evasive movements or a course of conduct that manifests an intent to cross a state or national border and violate the FFA."
United States. Department of Justice: http://www.justice.gov/