A nuclear physicist at a major international atomic research centre has been arrested as a suspected Al Qaeda terrorist.
He is a researcher at the European Organisation for Nuclear Research, known as CERN, in Switzerland.
The suspect is the elder of two brothers, aged 32 and 25, arrested in Vienne in South Eastern France on Thursday.
French intelligence sources said he was thought to have links with Al Qaeda's North African wing and was suspected of suggesting European targets for terror attacks.
CERN, on the French and Swiss border outside Geneva, is the site of the world's biggest particle collider, which aims to re-create the conditions of the Big Bang.
A £6billion experiment will collect data on the high-energy collision of protons that may show on a tiny scale what happened one-trillionth of a second after the massive explosion 14billion years ago that many believe formed the universe.
But some sceptics fear the impact of the particles could endanger the earth by creating micro black holes.
CERN and leading physicists have dismissed the Doomsday scenario and insist the project is safe but the arrest sent shockwaves through the scientific community last night, raising fears terrorists could be targeting the nuclear industry.
'The inquiry will doubtless say what were the objectives in France or elsewhere and indicate perhaps that we have avoided the worst possible scenario,' Bruce Hortefeux, the French Interior Minister said last night.
But CERN downplayed the threat, saying: 'His work did not bring him into contact with anything that could be used for terrorism.'
The suspect is said to have been working as a physicist with the Large Hadron Collider since 2003.
'He was not a CERN employee and performed his research under a contract with an outside institute,' said a spokesman.
'CERN is a particle physics research laboratory whose research addresses fundamental questions about the universe.
'None of our research has potential for military application and all our results are published openly in the public domain.'
'CERN is providing the support requested by the French police in this inquiry,' she added.
The experiment the suspect was involved in is the smallest of a series of installations along the 17-mile circular underground tunnel.
As many as 7,000 scientists from 20 countries are involved in the particle experiments.
The Large Hadron Collider shut down just nine days after it was launched last September after it overheated.
It is expected to start collecting data later this year or early in 2010.
Al Qaeda in the Islamic Maghreb, the terror network's north African group, has claimed responsibility for a suicide bombing at the French Embassy in Mauritania that wounded three people in August.