My (possibly flawed) understanding is that "classified information" in the US is pretty much a federal government thing, and is usually used for information relating to national security or spy-agency type stuff.
I would imagine information about informants or people with hidden identities would be considered privileged information in whatever state/local law enforcement jurisdiction created it, but penalties for leaking or distributing it would be a local matter, and many localities might not have specific laws on their books to deal with it.
Regarding on-going investigations, police aren't supposed to publicly discuss information about investigations, but they may if they deem that there is a public interest in doing so, or that doing so will help them with their case. I may very well be wrong here, but my gut suggests that in most places in the US there are likely not specific laws against public disclosure of details of ongoing investigations.
I would imagine information about informants or people with hidden identities would be considered privileged information in whatever state/local law enforcement jurisdiction created it, but penalties for leaking or distributing it would be a local matter, and many localities might not have specific laws on their books to deal with it.
Regarding on-going investigations, police aren't supposed to publicly discuss information about investigations, but they may if they deem that there is a public interest in doing so, or that doing so will help them with their case. I may very well be wrong here, but my gut suggests that in most places in the US there are likely not specific laws against public disclosure of details of ongoing investigations.