ICC lawyer Melinda Taylor and three associates have been under house arrest for more than a week in Zintan, where Gadhafi's son, Saif al-Islam Gadhafi, is being held by militia fighters, The New York Times reported Friday.
The militia accused the four, sent by the ICC to Gadhafi under an arrangement with his captors, of having suspicious documents and a camera disguised as a pen.
"Ms. Taylor had brought letters for [Saif] from two different people, dangerous people who are supporters of the old regime, and she had a page with drawings that looked like codes," said Ahmed al-Gehani, a Libyan lawyer traveling with the group as the government's liaison official.
Gehani said Taylor had tried to leave a meeting with "three blank pages that were signed by [Saif]" that "raised suspicions."
The militia captured the younger Gadhafi in the desert in November and has held him while awaiting an agreement on where he would be tried on numerous charges.
The detention of Taylor and her colleagues caught the ICC, based in The Hague, Netherlands, by surprise, the Times said.
Judge Sang-hyun Song, the court's president, issued a demand that the group be released immediately.
"These four international civil servants have immunity when on an official ICC mission," he said.
The ICC indicted Gadhafi for crimes against humanity committed during protests against his father's rule before the regime fell last year.
Moammar Gadhafi was killed in October.
Libya's new government said it wants to try Saif al-Islam Gadhafi in Libya instead of The Hague, but the court said it couldn't drop its case until Libya demonstrates it would prosecute him fairly.