After Moncef Kartas, a well-respected expert on illegal arms shipments into Libya, had cleared security at the international airport in Tunisia in late March, several plainclothes security officers were waiting for him in the arrivals hall, according to his defense team.
Few people knew of Kartas' travel plans, which had changed last-minute. But the officers "were clearly expecting him," Kartas' lawyer, Sarah Zaafrani, told DW.
Ever since, he has been imprisoned on charges of obtaining national defense secrets and passing them on to an unnamed "foreign state or its agents," according to the English translation of the official decision to open an investigation.
Espionage is a charge that can carry the death penalty in Tunisia, although it has not been enforced for several years.
The arms expert also stands accused of "willfully disclosing information gathered through interference, interception and audio surveillance."
UN: Arrest a clear rights violation
Kartas' arrest came as a complete surprise to Zaafrani and many of his friends and colleagues with whom DW spoke. A dual German-Tunisian national, Kartas is a member of the UN's Panel of Experts tasked with investigating possible arms shipments to and from Libya. The reported shipments occurred in violation of an embargo imposed in 2011 as the country descended into the violence that eventually led to the overthrow of longtime dictator Muammar Gadhafi. As such, he enjoys diplomatic immunity from arrest.
While states can ask the Security Council to lift immunity, Tunisia chose not to do so in this case.
Rather, the Tunisian authorities arrested Kartas, who was travelling with an official laissez-passer document issued by the UN. In doing so, Tunisia clearly violated his immunity, according to the UN, the German Foreign Office, and experts in international law contacted by DW.
Germany in back-channel talks
The event triggered an ongoing diplomatic standoff with the UN, which has called the detention a "very serious matter." Four days after the arms expert's arrest, UN Secretary-General Antonio Guterres raised the matter with Tunisian Prime Minister Youssef Chahed.
The arrest also angered the German Foreign Office, which is applying pressure behind the scenes. "We are doing all we can," one source told DW. German Embassy staff in Tunis, however, were only granted access to Kartas last week.
Germany heads the UN's Libya Sanctions Committee and agrees with the UN that Kartas enjoyed diplomatic immunity when he entered Tunisia. On Wednesday, UN spokesperson Stephane Dujarric reiterated the body's call for Tunisia to free Kartas, saying it had "requested Mr. Kartas' immediate release and for the charges against him to be dropped." He added that the UN "remains very concerned by the continued detention and prosecution" of its weapons expert.
Why the Tunisian authorities took such a step, which has drawn both the UN and Germany's ire, is up for debate. In Tunisia, there is no shortage of differing theories: "He clearly angered someone," Zaafrani told DW, adding that the accusations were "fabricated."
A key regional hub
What is undisputed is that Kartas was looking into the illicit trade of weapons to and from a country that has descended into turmoil and bloodshed — and has drawn important regional and international actors into the fray.
The Libyan quagmire has pitted the UN-backed Government of National Accord (GNA), based in Tripoli, against the self-styled Libyan National Army of strongman Khalifa Haftar.
The United Arab Emirates, Saudi Arabia and Egypt are seen as key supporters of Haftar. And the United States may have shifted their support to the Libyan National Army: In mid-April, US President Donald Trump issued a statement recognizing Haftar's "significant role in fighting terrorism and securing Libya's oil resources."
Amidst this political and security vacuum, the so-called "Islamic State" (IS) and other extremist groups have gained an important foothold in Libya. In 2016, the UN's Panel of Experts reported that foreign fighters used Tunisia and other countries as a transit point when travelling to Libya, and that IS had established links with illicit networks in order to smuggle fighters and arms across the Tunisian-Libyan border. The experts also noted that the vast majority of weapons used by terrorists in Tunisia originated in Libya.
Fuel and persons are also trafficked across the porous border. The Panel has repeatedly urged the UN's member states to abide by the embargo.
On the basis of its findings, the UN Security Council has issued travel bans and frozen assets of several Libyan politicians and strongmen.
Read more: The battle for Tripoli
Friends, family, and experts baffled by charges
Moncef Kartas was, according to his brother Adel, deeply invested in his work. "He took it very seriously. He was idealistic about his research."
"He's just not someone who would do something like that," Adel Kartas, a medical doctor in the United Kingdom, told DW on the phone, referring to the accusation of espionage. "It's just not believable."
Researching in this environment was not without risks, the elder brother conceded. "Of course it's not a job in insurance," he said with a chuckle. But, he added in a more serious tone, he had always thought his brother was safe because he was working for the UN.
As the weeks of his detention drag on, Adel Kartas said, his brother is trying to maintain hope that he will be released soon. "But of course it's demoralizing."
DW spoke to multiple sources, but even off-the-record many remained equally baffled by the accusations. Some pointed to pressure applied by important foreign actors; others speculated that Kartas' research could have implicated Tunisian business interests with links to the government.
But all agree it is unlikely that he will be released any time soon.
And the timing of his arrest on March 26, about 10 days before Haftar launched his offensive on the Libyan capital Tripoli, has raised some eyebrows. It came just shortly before the UN's Panel of Experts was set to issue an interim report to the Security Council.
Zaafrani, who as his lawyer has regular access to Kartas and provides him with food and medicine, said that his official questioning focused almost exclusively on his work on Libya.
But Wolfram Lacher, a Libya researcher at the German Institute for International and Security Affairs (SWP), told DW that the links to Kartas' research into arms smuggling may be overrated. "It's just speculation," he said. Lacher, who calls Kartas a "friend and respected colleague," added that the violation of the arms embargo by the United Arab Emirates and others were well documented, not just by the Panel of Experts. "Everyone knows that it's happening."
"I can't think of any plausible explanation why this happened," Lacher told DW.
DW was unable to verify any of the charges against Kartas, and numerous requests to Tunisian authorities have been left unanswered.
Radio device in question
So far, according to his defense team, the case seems to rest on a device in the researcher's possession — an RTL-SDR radio scanner — which Tunisian authorities say he used without special permission. The scanner is a small device often used by amateur plane spotters, which can be purchased online for around $30 (€27).
Zaafrani explained that her client used the scanner to track airplanes and follow which ones had turned off their flight radars, possibly indicating that they had flown into Libyan airspace. The device is not, the lawyer said, "the sophisticated spying tool that the Tunisian authorities are making it out to be."
A chilling message
Whatever is behind Kartas' arrest, it has had "a negative impact on the Panel's work," a spokesman for the UN told DW: "Given the uncertainty surrounding the arrest and continued detention of Mr. Kartas, all travels of the Panel to Tunisia have been suspended for the time being."
Tunisia is an important hub both for the Panel and other researchers and nongovernmental organizations dealing with Libya. High-level political and diplomatic meetings and seminars are often held there, important members of the Libyan diaspora reside in Tunis, and the country is a transit point for travel to neighboring Libya.
The arrest has also has sent a chilling message to other researchers. To some, it recalls the dark days under the dictatorship of former Tunisian President Zine El-Abidine Ben Ali, whose ouster in early January 2011 was a turning point in galvanizing the Arab Spring.
"If they can arrest someone with immunity, they can arrest anyone," one man told DW. "We should all be afraid."
Mabrouka Khedir and Moncef Slimi contributed to this report.