How to Address a (Special) FBI Agent

FBI are not police officers.
how about mr/ms smith.
in my experience v few people, outside of old films, address our police as officer.
maybe legal profession do, in court, but not the general populace.

On reading the title, did anyone else hear a spectral radio playing "tum, tiddle-um,tiddle-um, tiddle-um tim-tum tim tum-tu-tu-u-um ta-ra-a-a-ara rum -Dick Barton, Special Agent!"

On NCIS they are referred to as Agents aren't they but that isn't the FBI. But in the great scheme of things does it really matter? If you are in the UK why not approach the American Embassy??

are you writing a book/screenplay, OP ?

I don’t know location of OP but we have not had police officers or a station for nearly fifteen years. So it would be a case of spot the police officer!
FBI agents really?

They are called Agents or Special Agent. Agent Smith or Special Agent Johnson, etc.

They are not police officers, though the FBI does have its own police force.

Also, here innthevUS we call police officers Officer, unless they have a higher rank. Then it would be the rank.

Officer Smith
Sergeant Johnson
Captian Grant

I should think the answer is
1) extremely politely
and
2) maybe “Sir” ?

Actually, now that I think about it. A police officer works for a police department. That usually means city enforcement. Police chief is appointed by elected city officials.

A deputy sheriff works for the Sheriff's department, that means county. A Sheriff is elected by the people.

So if it is a police officer, you address them as officer. If it is a deputy sheriff, you address them as deputy.

Ranks are similar in both departments.

The term Agent is used at the national level for departments such as the FBI, CIA, DEA.

Special Agent means they have the authority to arrest subjects.

None of the national departments have the authority to over ride, or take over, local enforcement.

Agent Smith. They are agents of the Federal government but I'm not clear about US Marshals.