Australian lawyer Melinda Taylor and three colleagues from the International Criminal Court were arrested after meetings with war crimes suspect Saif al-Islam Gadhafi, son of late Libyan leader Moammar Gadhafi.
Taylor is alleged to have passed documents to Saif al-Islam from one of his supporters. Nassar el-Manee, a spokesman for the interim government in Tripoli, described the documents as a threat to national security.
"An interrogation of them is under way," he was quoted by The Guardian newspaper in London as saying. "There is evidence that proves they have breached the law."
Australian Foreign Minister Bob Carr met with this week in Tripoli with Libyan authorities to try to secure the release of the ICC staff members. Her fellow staff members were told they were free to go but are staying with the Australian lawyer on a voluntary basis.
Tripoli has called for the ICC to apologize for the incident. The ICC maintains its lawyers are protected by immunity.
Libya next month has its first democratic election in a generation. The vote comes amid a backdrop of violence as the BBC reports more than 100 people were killed during tribal clashes in the west of the country.