Bow Street runner
FBI
FBI agent
Federal
G-man
Internal Revenue Service
Secret Service
Sherlock Holmes
T-man
assessor
customhouse
customs
detective
exciseman
farmer
fed
hotel detective
house detective
house dick
inquiry agent
investigator
narc
operative
plainclothesman
police detective
private detective
private investigator
publican
sleuth
store detective
tax assessor
tax collector
tax farmer
taxer
taxman
treasury agent
government agent