Scientific hair analysis is being used to verify the story of a German national who says he was abducted while on vacation in Macedonia and taken to a prison cell in Afghanistan.
Mr Masri claims he was imprisoned in Kabul
Lebanon-born Khaled el-Masri claims he was abducted on suspicion of terrorist activities, drugged, put in a plane and flown to Kabul. There, he was interrogated and accused of working for Osama bin Laden.
German authorities - who are taking the claims seriously - have asked forensic scientists to test whether his hair can reveal if his story is true.
"If you take a hair sample from an individual, and then section it centimetre by centimetre, you can then analyse for various naturally-occurring isotopes in that hair," Dr Stuart Black, a senior lecturer in environmental radioactivity at the University of Reading, told BBC World Service's Science In Action programme.
"That will give you some indication of where that individual has been living over the past few months."
Macedonian border guards allegedly seized and imprisoned Mr Masri, before flying him to Afghanistan.
He says he was detained for five months as a terrorist suspect. He alleges that while he was being held by the Afghans, it was at the behest of America. He claims he was then flown back to Europe and left in Albania.
Disorientated and lost, he was found walking close to the Albanian border - where he was detained as an illegal immigrant, as his passport had no Albanian stamp.
On his return to Germany, he found his family had left the country. He is now seeking answers for treatment he considers "criminal and humiliating".
Samples from his hair may prove Mr Masri's story
The German authorities are putting his hair to the test for traces of being in those countries.
"There are small particulates in the air, which contain naturally-occurring materials from rocks and soils blown up in the wind," Dr Black explained.
"Those are all indicative of where you have been."
Dr Black added that it would be "perfectly possible" to tell whether Mr Masri's claims were true or not - at least on a "country by country basis".
Hair samples are sectioned in the lab centimetre by centimetre - as hair grows at approximately 1cm a month, depending on metabolic rate.
Each section is then dissolved in acid and analysed by a mass spectrometer - a machine that identifies forensic samples by examining their molecular mass.
As well as the air, isotopes from Afghanistan's water would also be detectable.
"Those things will be picked up and deposited into your hair," Dr Black added.
"There will be a signal within you hair."