Missouri preparing for news conference on Ferguson decision -- but time and date unknown
People march in Washington on Sept. 6, 2014 to protest the killing of black teen Michael Brown in Ferguson, Missouri. (Nicholas Kamm/AFP)

Prosecutors are preparing a news conference to announce the decision of the grand jury weighing whether to bring charges against a white police officer who shot dead an unarmed black teenager in Ferguson, Missouri, officials said on Friday.

The grand jury was still in session on Friday and no date or time for the decision announcement was known, the St. Louis County Prosecuting Attorney's Office said. Officials have said a decision by the grand jury is expected by the end of the month.

U.S. Attorney General Eric Holder urged police to show restraint in dealing with any protests that may follow the grand jury's decision on whether to indict the officer, Darren Wilson, in the Aug. 9 shooting of 18-year-old Michael Brown.

Legal experts say the grand jury can consider charges ranging from first-degree murder to criminal negligence.

Groups from across the country have said they would take to the streets again in large numbers if charges were not brought, prompting local police to draw up contingency plans for possible fresh violence.

Missouri Governor Jay Nixon has already declared a state of emergency and called in National Guard troops to back up local police in anticipation of protests.

As tensions simmered in the St. Louis suburb over a case that has become a flashpoint for U.S. race relations, police in riot gear arrested three people in overnight protests that led to scuffles, St. Louis County police said. Police said they doused one demonstrator with pepper spay for resisting arrest.

Lawyers for Brown's family say he was trying to surrender when the officer shot him. Wilson's supporters say he shot Brown in self-defense.

Holder said the Justice Department was providing new guidance to law enforcement authorities about how to approach maintaining public safety while safeguarding the free speech rights of protesters.

"The Justice Department encourages law enforcement officials, in every jurisdiction, to work with the communities they serve to minimize needless confrontation," Holder said in a video address released by the department.

Holder also sent a message to protesters that "the most successful and enduring movements for change are those that adhere to non-aggression and non-violence."

Ron Davis, the top official in a Justice Department office advising Ferguson authorities on crowd response, told Reuters the department has been pressing local police not to use the heavy-handed tactics that spurred anger in August. Local police used military vehicles and riot gear to deal with some protesters in August.

(Additional reporting by Emily Stephenson in Washington; Writing by Jon Herskovitz; Editing by Will Dunham)