In Illinois v. Gates the USSC adopted the totality of the circumstances test for informant's information justifying a warrant. The police received an anonymous letter that was used to justify a warrant. The totality test allowed it because many of the items in the letter were verified, but the informant was never known or their reliability tested.
So an anonymous letter that was accurate in some respects justified a warrant.
What if the letter had come from the police to the police? Let's say I'm a zealous cop who can't get a warrant for my case. I write my department an anonymous letter with information that can be verified (like in Gates) and with enough incriminating information to justify the warrant I want.
Gates seems to encourage this.
Thank goodness Tennessee still uses the Agular/Spinelli test.