Typically used for this. Google it and you will see that practically every state has an office with this title.

--------------------------------------------------
Note added at 11 hrs (2012-01-23 20:45:23 GMT)
--------------------------------------------------

And of course we should not forget the FBI :)

--------------------------------------------------
Note added at 1 day8 hrs (2012-01-24 17:30:11 GMT)
--------------------------------------------------

Here is some more info to help in deciding...

http://en.wikipedia.org/wiki/State_bureau_of_investigation
"SBIs can also exist either independently or within a Department of Public Safety (which is an umbrella agency coordinating and/or controlling the various state level law enforcement agencies) or a state police force (which is a general law enforcement agency)."

Even if the Iranian police organization is different, "bureau" and "investigation" are collocates in this context...see http://corpus.byu.edu/coca/