Ferguson braces for protests ahead of Darren Wilson grand jury decision
Missouri Governor Jay Nixon (C) stands during a press conference held to discuss security concerns when the grand jury's decision in the Michael Brown case is announced on November 11, 2014 in Weldon Springs, Missouri (AFP Photo/Scott Olson)

Authorities and residents in the US city of Ferguson are bracing for a decision about whether a police officer who fatally shot an unarmed black teen should be prosecuted.

A grand jury is looking at the racially charged shooting, in which white police officer Darren Wilson shot 18-year-old Michael Brown. The jury is expected to announce its findings any time between mid- to late-November.

"Quite simply, we must and will be fully prepared," Missouri Governor Jay Nixon cautioned at a news conference on Tuesday.

The August 9 shooting led to weeks of violence in the Missouri town. Authorities and business owners are preparing for potential fresh clashes between law enforcement officials and protesters.

Nixon said more than 1,000 police had undergone extensive specialized training over the past two months. During the clashes, looters smashed storefronts and set fire to businesses.

"In the days immediately following Michael Brown's death, peaceful protests were marred by senseless acts of violence and destruction," Nixon said.

The governor said police and National Guard troops from around the region were on standby in case of any new problems.

"This is America. People have a right to express their views and grievances, but they do not have the right to put their fellow citizens or their property at risk," Nixon said.

Some US media including CNN have reported a surge in gun and ammunition sales, as residents and business owners seek to protect themselves in the event of renewed violence.

Brown was shot at least six times and his body was left in the street.

Wilson claims he shot in self defense.

In the United States, grand juries are sometimes used to review a case before deciding whether criminal charges should be sought. The jury could indict Wilson, meaning he could face trial, or determine there is no case for him to answer.

The shooting led to weeks of violence in the St. Louis suburb of 21,000, which has an African-American majority and an overwhelmingly white police department and town government.

Some demonstrators complained that police used undue force during peaceful protests.

Brown's death prompted a nationwide discussion about race and led to a Department of Justice probe into the Ferguson police department.