The phone belonged to San Bernardino County, which was quite happy to have the FBI get to work on it. Syed Farook had been authorized to use the phone, but only under the county's terms. If it had been Farook's phone, and if his surviving relatives asserted some claim, that might have made matters a little trickier.

But in this case, Farook is dead. He never owned the phone. The organization that did own the phone wants the FBI to crack it, for easily understandable reasons. On ownership, at least, the facts are unusually friendly to the FBI.

Most of the time, people can't kick down your front door, search your house, and pack up a bunch of your possessions without your permission. Start digging here - https://en.wikipedia.org/wiki/Police_power_(United_States_co...

There are really a few sub-questions: Is it legal, and are the results admissible as evidence?

Whether or not it's legal depends on the circumstances. There are many cases in which the police can search something, e.g., if they believe it's critical to prevent an immediate threat. Whether or not it's admissible is more thorny, and something that's still being fought out in the courts. (google: warrantless cell phone search admissible)

The phone belonged to his employer. The employer gave the FBI authorization to access to the phone.

There was an active warrant allowing it.

There are laws against breaking into people's homes, and yet the police still does it daily. IANAL, but I assume a warrant would apply to this case just the same.

If no prosecutor would ever touch it, is it really illegal?