It literally is at the discretion of the investigators. See 49 CFR 831.12(b):
"§ 831.12 Access to and release of wreckage, records, mail, and cargo
(a) Only persons authorized by the NTSB IIC may be permitted access to wreckage, records, mail, or cargo.
(b) Wreckage, records, mail, and cargo in the NTSB's custody will be released when the NTSB determines it has no further need for such items. Recipients of released wreckage must sign an acknowledgement of release provided by the NTSB."
So if the NTSB decides they don't need a piece of cargo, they have the option to release it from custody.
"§ 831.12 Access to and release of wreckage, records, mail, and cargo
(a) Only persons authorized by the NTSB IIC may be permitted access to wreckage, records, mail, or cargo.
(b) Wreckage, records, mail, and cargo in the NTSB's custody will be released when the NTSB determines it has no further need for such items. Recipients of released wreckage must sign an acknowledgement of release provided by the NTSB."
So if the NTSB decides they don't need a piece of cargo, they have the option to release it from custody.