Who investigates fires?

The police don’t do it, as I don’t believe there’s an “Arson” department in the police force ( I may well be wrong). Who does it, if it isn’t them, and why not the police? What kind of training/legal powers do they have? Are they the ones driving the “Fire Police” cars I occasionally see?

In Houston it’s the Fire Department’s Fire and Arson Investigation Division:

The Fire Department investigates fires.
Oh I see Ringo already posted that.

In that case.
What he said.