ST LOUIS - A woman allegedly shot herself in the head in a freak accident with a gun that she had bought with her boyfriend to protect themselves from unrest that may erupt from a grand jury's announcement on Tuesday on the killing of a black teenager by a white cop.

The woman, identified in a police report as 26-year-old Becca Campbell, was riding in a car last Friday night in downtown St Louis when the accident happened, the city's Metropolitan Police Department said.

Her boyfriend, said to be 33 years old and was not identified, allegedly told the police he was driving the car when Campbell waved the gun while joking about being ready for Ferguson, the CNN reported, citing unnamed sources.

As the boyfriend ducked to avoid being pointed at by the weapon, he got distracted and rammed into another car. The gun went off in the jolt, striking the victim in the head with a bullet and killed her, according to the sources.

Police are investigating the boyfriend's claims.

The death of 18-year-old Michael Brown, a resident of a predominantly black city of Ferguson with a white-dominated power structure, sparked weeks of sometimes violent protests in the St Louis suburb and has become a flashpoint for strained US race relations, the Reuters reported.

Prosecutors are set to announce at 8pm local time (0200 GMT, 10am Singapore time, Tuesday) on Monday the grand jury's decision on whether to criminally charge the white police officer, Darren Wilson, for the Aug 9 fatal shooting of Brown.

Missouri Governor Jay Nixon has called up the National Guard, and local officials have been planning a massive police presence to quell any potential violence, particularly if Wilson is not indicted.