A lawyer from a four-member International Criminal Court (ICC) delegation has been held in Libya after meeting Saif al-Islam Gaddafi.
The lawyer, understood to be an Australian named Melinda Taylor, was held after visiting Saif al-Islam in the western town of Zintan.
She is accused of trying to smuggle documents to the detained son of deposed leader Col Muammar Gaddafi.
The ICC and Libya both want to try him for crimes under the former regime.
Ms Taylor and at least one other colleague are being investigated for trying to pass on the documents to Saif al-Islam during a visit on Friday.
"She is not in jail. She is being detained in a guesthouse, her colleagues are with her," Ahmed el-Jehani, the Libyan representative to the ICC, told reporters.
Members of the brigade holding Saif al-Islam say they discovered documents including a letter from a former confidante of his who is now in Egypt, the BBC's Rana Jawad reports from Zintan.
The brigade commander said the transitional authorities in Tripoli had requested the release of the ICC officials, but the two would remain in detention in Zintan - "where the crime has been committed" - pending an investigation by the attorney general's office.
Reporters were shown the documents from a short distance, but were not allowed to view them in person, our correspondent says.
Saif al-Islam, who was captured last November by militiamen as he tried to flee the country, has been indicted by the ICC for crimes against humanity.
Libya's interim government has so far refused to hand over Saif al-Islam for trial in the Netherlands - seat of the ICC.
Libya has insisted he should be tried by a Libyan court.
