Law enforcement agencies across Contra Costa County began shielding their radio chatter from the public this week, ending decades of transparency and closing shut a key window into crime across the region.

Police and sheriff’s deputies started encrypting — or making secret — conversations between themselves and dispatchers before dawn Tuesday. Publicly-available feeds for the Concord and Pittsburg police departments, as well as for parts of the Contra Costa County Sheriff’s Office, went silent between the hours of 1 a.m. and 5 a.m. that day, suggesting the agencies had turned to private channels that are only accessible by other law enforcement officers.

In an unexpected turn, numerous agencies across Alameda County did not follow suit as planned Thursday — raising questions about the vi

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