Nouns ending in -teur are usually masculine.
The suffix names a person or thing that performs an action.
Is enquêteur masculine or feminine?
l'enquêteur
The rule behind it
95.8% of -teur nouns are masculine
In context
Dan était un enquêteur de la brigade scientifique.
Dan was a crime scene investigator.
Les enquêteurs ont fouillé la voiture de Tom.
The investigators searched Tom's car.
L'ubiquité apparente du personnage intriguait les enquêteurs.
The person apparently being in more place than one was intriguing to the investigators.