Missouri authorities are drawing up contingency plans and seeking intelligence from U.S. police departments on out-of-state agitators, fearing that fresh riots could erupt if a grand jury does not indict a white officer for killing a black teen.

The plans are being thrashed out in meetings being held two to three times a week, according to people who have attended them. The FBI said it was also involved in the discussions.

Details of the meetings and intelligence sharing by Missouri police agencies and their counterparts in other parts of the country have not been reported before.

The grand jury is expected to decide next month whether to bring criminal charges against police officer Darren Wilson, who shot dead Michael Brown, 18, on Aug. 9 in Ferguson, Missouri.

In differing accounts, police have said Brown struggled with Wilson before the fatal shots were fired. But some witnesses say Brown held up his hands and was surrendering when he was shot multiple times in the head and chest.

If charges are not brought against Wilson, police fear an outbreak of violence not just in the St. Louis suburb of Ferguson, but across the greater metropolitan area and even in other U.S. cities, according to St. Louis County Police Chief Jon Belmar and others involved in the planning meetings.

James Knowles, the Ferguson mayor who has been attending the meetings, told Reuters the fear is that if violence is triggered by the grand jury decision, "the unrest is going to be far beyond the city of Ferguson." (...)