Originally Posted by Deeg

Many federal law enforcement agencies have policies on when their employees are allowed to use state law for "citizen's arrests" and still be considered within the scope of their duties. In fact, federal agents occasionally have to rely on such laws to make non-federal arrests in states which do not grant them peace officer status.

So...I can't address whether they are credible or permissible. But they do happen. And they are encouraged by the federal government under certain circumstances.