Unlock checked luggage for plane travel in USA ?

Is it true u must unlock checked luggage when travelling in the US ?
Or it is when u arrive from abroad only ?
This is supposed to prevent custom breaking into it...

no I don't think so, going the other way TSA might have to get
*into* your luggage
you can buy travel locks that TSA has keys for..

No you dont have to unlock checked luggage. And it should be automatically insured to prevent that from happening.

Transport Security Administration (TSA) in the US routinely opens and inspects luggages on all flights to/from/within USA. If your bag has been opened, you will get a note from the TSA inside your luggage to inform you that they have opened your bag for inspection. If TSA has sampled your bag to be opened and they cannot open your bag, they may force open and destroy the locks.

You can either leave your luggage unlocked, or use can use TSA-approved locks which they can unlock and re-lock. In some airports, you can actually get your unlocked luggage strapped with the same material used in strapping boxes and crates in cargo shipments.

For details of TSA-approved locks:

http://www.tsa.gov/travelers/airtravel/assistant/locks.shtm