(AP) -- A grand jury has begun hearing evidence as it weighs possible charges against the Ferguson police officer who fatally shot 18-year-old Michael Brown.

St. Louis County prosecutor's spokesman Ed Magee confirmed the panel heard evidence Wednesday but offered no other details. Officials have vowed not to release any information about the grand jury's proceedings in order to avoid compromising the case.

During the day, a small group of protesters gathered outside the suburban St. Louis building where the grand jury was meeting. About 20 police officers stood outside the building's front entrance, which was also blocked off by yellow police tape.

It could be weeks before the grand jury decides whether Officer Darren Wilson should stand trial for Brown's Aug. 9 death.