The Missouri authorities leading the investigation into the shooting of an unarmed black teenager by a white police officer are increasingly facing questions about whether they can be impartial.

Grand jurors will be hearing evidence over the coming weeks that will help determine whether state charges will be brought against Ferguson officer Darren Wilson for the Aug. 9 shooting of 18-year-old Michael Brown.

The case is being overseen by St. Louis County Prosecutor Bob McCulloch.

But some local residents and prominent black politicians have been calling on McCulloch to step aside and allow a special prosecutor to take over. They believe McCulloch, who is white, will favor the officer.

Special prosecutors have been used in some high-profile cases elsewhere.

McCulloch is pressing forward and defending his ability to be fair.