The governor of Missouri has activated the state's National Guard in anticipation of a grand jury decision over the killing of an unarmed black teenager.
In a statement, Jay Nixon said the guard will "support law enforcement's efforts to maintain peace".
A panel is deciding whether to charge Officer Darren Wilson in the August death of Michael Brown, 18. No specific date for the jury's decision has been given.
Brown, 18, was shot and killed by Mr Wilson after a confrontation in Ferguson, a majority-black suburb of St Louis. Witnesses say Brown had his hands up in apparent surrender to the officer when he was shot.
Police have said there was a struggle between the teenager and the officer before the shooting. His death set off violent protests and the police and other law enforcement were criticized for being heavily armed and using excessive force.
Mr Nixon signed the state of emergency order on Monday.