I've watched Sicario a few days ago and I've quite enjoyed it, especially the way in which the plot is gradually and simultaneously revealed to the audience and to the female protagonist, Kate Macer (Emily Blunt).
As far as I understand, the hidden scope of the american "task-force" led by the CIA agent Matt Graver (Josh Brolin) is to take out the head of the Mexican drug cartel with the help of the hitman Alejandro (Benicio Del Toro) in order to push as far away as possible from the US borders the drug cartels with their load of merciless violence, at the cost of restoring the power to the infamous Colombian cartel of Medellin.
It is made quite clear in the movie that an FBI agent (Macer) must absolutely be a part of the plan. For instance at the end of the movie Macer is forced at gunpoint by Alejandro to sign a document that claims that everything in the mission has been done by the book.
But what I don't understand is why the mission must be "sanctioned" by Macer. Is it because as an FBI agent she has certain "prerogatives" that a CIA agent doesn't possess? What is the official role of Kate Macer in the movie?