Tunisia's arrest of an expert investigating possible violations of an arms embargo on Libya has sparked a diplomatic standoff between Tunis and the United Nations.
Moncef Kartas, a member of a UN panel of experts, has been held for five weeks on suspicion of spying -- charges that could carry the death sentence.
The UN insists Kartas, a Tunisian-German dual national, has diplomatic immunity and has demanded that authorities reveal the reasons for his detention on arrival in Tunis on March 26.
On Tuesday, his lawyers submitted an official request for his release.
They noted that a key pillar of the charges against him was that he had "a device giving access to public data on flights of civil and commercial aircraft," his lawyer Sarah Zaafrani said, according to Agence France Presse.
On Tuesday, a group of researchers published an open letter in several European newspapers, demanding his immediate release.
"The detention of Moncef Kartas on false grounds and in violation of his immunity raises serious questions about the rule of law in Tunisia," wrote the group of around a hundred academics and researchers.
They said that "not a single piece of evidence" had been released to justify his detention.
The prosecution said last month it had issued an arrest warrant over an inquiry into "the acquisition of security information related to the fight against terrorism and the dissemination of this information in violation of the law".
The interior ministry said it had seized documents containing information that could harm "national security", along with banned communications equipment.
The expert is likely to be held in prison throughout the investigation, which could last several months.