Legal question regarding client safety

I had a client the other day tell me a story about how he was once almost pulled into a tangle with LE because his provider was arrested. An officer called him from her phone, told him what happened, and then said the usual: We already know what you've done, it's best if you come down and confess,, etc. etc.

He was at a party at the time, and quickly pulled aside his lawyer friend to ask him what to do. The lawyer said to call the officer back and ask for a "Letter of _____", which is a letter of immunity in exchange for testimony against the accused. "Letter of ______" was the magic phrase, and the officer said never mind and hung up.

This is really great info to share, if only I could remember the legal term! Help?
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