FERGUSON, Mo. — After a week of violent clashes between police and protesters, Missouri authorities leading the investigation into the police shooting death of an unarmed black teenager are increasingly facing questions about whether their eventual findings can be seen as credible among residents who are highly distrustful of those in charge.

The coming days and weeks will be crucial as grand jurors began hearing evidence that will help determine whether Ferguson police officer Darren Wilson is charged with a state crime for the Aug. 9 shooting of 18-year-old Michael Brown.

The U.S. Justice Department is conducting a separate civil rights investigation, which could mean there are two decisions about whether to charge Wilson, who is white.

The state’s case is being overseen by St. Louis County Prosecuting Attorney Bob McCulloch, who is white, and remains in charge despite mounting pressure to step aside from some local residents and black St. Louis area officials who believe he cannot be impartial.

In some other prominent cases — most notably, the 2012 racially charged shooting of Trayvon Martin in Florida — special state prosecutors have been appointed to determine whether to pursue charges. That sometimes has occurred only after the local authorities took no action.