The four-member ICC team has been held in Zintan since June 7, accused of carrying a pen camera and attempting to give Seif al-Islam a coded letter from his former right-hand man, Mohammed Ismail, who is on the run.
"The information reported by the Libyan authorities will be fully investigated in accordance with ICC procedures following the return of the four staff members," The Hague-based court said in a statement.
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"When the ICC has completed its investigation, the Court will ensure that anyone found responsible for any misconduct will be subject to appropriate sanctions," it added.
The ICC wants to try Seif al-Islam, 39, for crimes against humanity during his father's rule. Tripoli insists he should be tried locally and filed on May 1 a motion challenging the ICC's jurisdiction to put him on trial in The Hague.
The ICC team held in Libya includes Australian Melinda Taylor.
Australian Foreign Minister Bob Carr suggested on Tuesday after travelling to Libya the previous day that a simple "apology for inadequate consultation on protocol and procedures" could speed up the four's release.
