My understanding is that the FBI wanted to see which, if any, third party messaging services the phone's owner was using and his usernames so they could serve a subpoena to those services.
The information might not be admissible to implicate someone, but it should still count as probable cause (IANAL). So they could legally use it to then search whoever may be implicated, and any evidence from that should be admissible.
There's no claim that they violated any rights searching the phone (which after all belonged to them), but that the process may be unreliable. That means it's still enough for probable cause, or that seems plausible to me. I don't know whether this is right.
>The most common of such usage is with reference to evidence, testimony, identification by witnesses, or confessions that have been obtained by law enforcement illegally.
>> "...the phone (which after all belonged to them)..."
Pretty sure the phone was the property of whomever was named as the dead guy's inheritor in his will. Absent a will it should have gone to the next of kin or probate court or similar. One place the dead guy's property should not go is to the FBI. That's not how property rights work.
I recognize that in practice the FBI will do as they damn well please, but in principle we do have laws for this.
> Technically, the iPhone in question (the one the FBI is demanding that Apple unlock) was purchased by the San Bernardino Department of Health. And as security researcher Christopher Soghoian has pointed out on Twitter, the Department tried to reset the phone’s iCloud password remotely in the hours after the attack. The department hoped to gain information from a possible back-up of the phone to iCloud. Instead, it rendered the account useless.
My point is that a warrant allowing seizure of evidence does not transfer legal ownership to the the FBI. If the county owned the phone, then fine. It remained the county's, not the FBI's, regardless of any investigative warrants.