Tripoli, Libya (CNN) -- The International Criminal Court demanded Saturday the immediate release of a lawyer and three other staff members who were detained while visiting the son of deposed Libyan dictator Moammar Gadhafi.
Defense lawyer Melinda Taylor was detained in the city of Zintan after she was discovered carrying documents and letters Libyan authorities said jeopardizes Libyan national security, said Ahmed Gehani, a Libyan lawyer who serves as a liaison with the criminal court in The Hague, Netherlands.
He said guards first confiscated a camera pen on Taylor and allowed her to go in to interview Saif al-Islam Gadhafi, who was captured in hiding last November.
A later search by female guards found documents and letters written by former regime members, Gehani said. They included a letter from Mohammed Ismail, Saif Gadhafi's former right-hand man.
Taylor was also carrying three blank papers signed by Saif Gadhafi, Gehani said.
Taylor is not in jail, but was placed under house arrest in Zintan, along with a translator. Gehani said Libyan authorities are looking into whether she was spying and communicating with the enemy.
The two other court members from the registry office were free to go but chose to remain with Taylor and the translator.
Gehani said a criminal court delegation is scheduled to arrive in Tripoli Sunday to try to resolve the situation.
Judge Sang-Hyun Song, the president of the criminal court, said the staffers have immunity while traveling on an official court mission.
"We are very concerned about the safety of our staff in the absence of any contact with them," he said.
Court spokesman Fadi El Abdallah said the court has had no contact with the four staff members. Nor has it received any official information from the Libyans as to why they were detained.
"We lost contact with them on Thursday, and since then we have had no opportunity to clarify the reasons for their detention," Abdallah said.
"When we got a hold of the Libyan authorities they only said that they would be released soon, but they didn't say the reason for why they had been detained," he said.
Both the International Criminal Court and Libya's new authorities want to put Saif Gadhafi on trial.
The criminal court has demanded that Libya hand him over immediately to face accusations of crimes against humanity. Libya appealed the decision, saying that he should be tried at home.
The criminal court issued an arrest warrant for Saif Gadhafi, who stands accused of having a role in the deaths of Libyans who protested his father's 42 years of dictatorship.
Taylor and lawyer Xavier-Jean Keita said in April that Saif Gadhafi has been mistreated and "physically attacked" since his capture.
In a strongly worded statement, the lawyers described Gadhafi as being in a legal black hole, held in total isolation except for visits from officials. He also suffers dental pain because he hasn't had treatment, and Libyan authorities have given him nothing to remedy the pain, the lawyers said.
CNN's Per Nyberg contributed to this report.