ferguson grand jury
A man walks past a burning police vehicle during clashes between police and protesters in FergusonJewel Samad/AFP

A Missouri woman accidentally shot herself in the head with a firearm she bought in preparation for protests in Ferguson, according to reports.

Becca Campbell, 26, fired the shot that killed her during a car crash in St Louis, police sources told CNN.

Campbell was a passenger in a vehicle driven by her 33-year-old boyfriend when the accident occurred, on Friday last week (21 November).

The man said they had recently purchased a gun, fearing that violent riots would have erupted following a grand jury decision, due on Monday, on whether to indict a police officer in relation to the shooting of local teenager Michael Brown.

Campbell pulled the gun as they were driving in central St Louis late on Friday night, and waved it inside the vehicle jokingly saying they were ready for Ferguson.

The man ducked to get out of harm's way but lost control of the vehicle, rear-ending another car.

The impact caused the gun to go off, eventually killing the woman. Police were reportedly probing the boyfriend's account.

On Monday, a Missouri grand jury decided not to indict police officer Darren Wilson in relation to the shooting of Michael Brown, sparking massive riots in Ferguson.