ministerial

1. adj. Relativo al ministerio o gobierno del Estado,o a algunos de los ministros encargados de su despacho:
orden ministerial.

See: http://www.wordreference.com/definicion/ministerial

In American English, this would be something like the State Trooper division, since Mexico (somewhat like the US) is divided into provincial governments called states (estados).

The State Police in Mexico has national jurisdiction, since not all municipios have police. See http://en.wikipedia.org/wiki/Policing_in_Mexico for a moderately informative treatment on the subject (alas, with not all Spanish terms listed).

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Note added at 6 days (2008-11-21 22:19:56 GMT)
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I concur that this refers to "a branch of the police that investigates crimes." Or, to be more specific, Mexican police are divided into judicial and investigative units. The context of the sentence given ("la policía ministerial no ha establecido el móvil del crimen.") makes it clear that this police unit determines motive. This makes it an investigative crime unit.

I don't think that State Trooper or State Police excludes this function. Perhaps 'Investigative Crime Unit' is the best term, if we really want to accentuate that ROLE of the police. The context does seem to warrant this.