BRUSSELS — Visitors need 24-hour security clearance, grizzled Belgian guards man the entrance, no phones or cameras please: welcome to the latest, and probably the last, of the big Balkans war crimes probes.

The US diplomat and prosecutor leading the investigation into one of the most gruesome and politically sensitive affairs of the times -- allegations of organ trafficking in the 1990s implicating Kosovo's current leadership -- has set up office not in the Balkans, but in Brussels.

Concerns about witness harassment and tampering of evidence are such that Clint Williamson, the 51-year-old American at the head of the European Union investigation, the most notorious inquiry to date, is taking no chances.

"Witness intimidation and witness protection is a big factor," he told AFP in an interview.

"We've gone to great lengths to put this investigation in a secure posture," said the former US ambassador-at-large for war crimes who in the 1990s worked as a trial attorney at the International Criminal Tribunal for the former Yugoslavia (ICTY) in The Hague.

At the heart of the matter are claims that ethnic Albanian rebels in Kosovo removed organs from Serb prisoners of war and sold them on the black market.

In the process, up to 500 people, mostly Serbs, were abducted, tortured and killed. These events took place in the chaos following the end of the Kosovo war in June 1999.

In a hard-hitting 2010 report, Council of Europe rapporteur Dick Marty alleged that senior Kosovo Liberation Army (KLA) commanders -- including current Kosovo Prime Minister Hashim Thaci -- were involved.

The report said organs were taken from the bodies of prisoners held by the KLA in Albania. It also linked the KLA to mafia-style crime. Thaci and his government have denied the accusations and condemned Marty's report.

Appointed to head the EU Special Investigative Task Force (SITF) set up a year ago to investigate Marty's charges, Williamson said that to avoid leaks, intimidation, or even worse, none of his team was from the western Balkans.

"This is a fully internationalised effort," he said. "We have taken steps to set up firewalls between us and other judicial bodies in Kosovo so it's very very autonomous and separated."

Why an American at the head of a European Union probe? Because he had 18 years in the Balkans and "a fairly unique background" both as war crimes prosecutor and as a diplomat able to work with governments "at the highest levels."

Williamson, who declined to give details on evidence compiled so far by his team of 20-odd prosecutors, investigators and analysts, estimated it would take another two years to complete an inquiry that continues to poison the region.

"It really is a dark cloud," he said. "Because these allegations have been levelled at the current prime minister of Kosovo and other individuals in high profile roles with the KLA, it has an impact on relationships in the region."

To help investigators access witnesses in Albania, believed to be the scene of mass graves and defunct medical operating theatres, Williamson and President Sali Berisha drafted a law adopted by parliament enabling the SITF team to work in the country.

In Kosovo itself, the EU rule of law mission runs judicial and police affairs. Serbia's new leaders too pledged to cooperate last month and Williamson recently travelled to Macedonia and Montenegro to seek judicial deals.

"It's important for us to speak to witnesses in a confidential fashion and we have to work this out with governments in advance," he said. "But we are also talking to them about the possibility of witness relocation."

"This is an issue that has been difficult for other high profile Kosovo Albanian trials at the ICTY. There have been strong indications of witness intimidation so we're very mindful of this."

Williamson said though he was "happy with the progress" there were "definitely a lot of challenges involved."

Crimes were 12 to 13 years old and evidence and witnesses spread across the region and beyond. "The passing of time is a problem."

But the real question, he said, was whether witnesses who talked to Marty in his role as a rapporteur would be willing to participate in a criminal investigation and testify in court.

"So it's hard to say whether our findings will be the same as what Marty produced," he said. "It's just much too early to determine if we'll come to the same conclusions. The problem will be to obtain information usable in court."