Yeah its illegal without permission of the other party.
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"A computer lets you make more mistakes faster than any invention in human history - with the possible exceptions of handguns and tequila." - unknown
If you are in law enforcement then as long as one party (the officer) is aware that there is a recording in progress then it's absolutely legal regardless if the other person does not know. Also, to make it legal, just state that a conversation may or may not be recorded at the beginning of the conversation, but in many states it's still admissable if at least one person in the conversation knows there's a recording going on.