Cern's particle collider aims to recreate conditions of the Big Bang
Anti-terrorism magistrates in Paris have filed preliminary charges against a French physicist arrested last week on suspicion of links to al-Qaeda.
The 32-year-old man of Algerian descent works at the European Organization for Nuclear Research (Cern), Europe's main centre for studying particle physics.
He was one of two brothers detained in the town of Vienne on Thursday.
Officials said he had been in contact with people linked to al-Qaeda in the Islamic Maghreb and planned attacks.
The researcher had been working on an experiment at the Cern laboratory which houses the giant Large Hadron Collider, designed to recreate the conditions that existed just after the Big Bang. It is located on the Swiss-French border.
After his arrest, Cern was quick to stress that his job "did not bring him into contact with anything that could be used for terrorism".
On Monday, spokesman James Gillies said the man had not been seen at work for most of the year because he was ill, but that he had been in touch via e-mail.
The scientist, who has so far not been identified under French judicial rules, appeared at a court in Paris to face charges of "criminal association with a terrorist enterprise", officials said.
He was placed under formal judicial investigation and would later hear whether he would have to remain in custody until trial, they added.
His 25-year-old brother has been released without charge.
After their arrest last week in Vienne, about 100km (65 miles) south-west of Cern's laboratory, judicial sources told the AFP news agency that agents had come across him during a separate inquiry into the recruitment of would-be Islamist militants who wanted to fight in Afghanistan.
The physicist had exchanged messages over the internet with people known to be close to al-Qaeda in the Islamic Maghreb and expressed a desire to carry out attacks, but had "not got to the stage of carrying out material acts of preparation", one said.
On Friday, French Interior Minister Brice Hortefeux said magistrates would "doubtless establish what the targets were in France or elsewhere, and perhaps indicate that we have avoided the worst".
The BBC's security correspondent, Frank Gardner, says the prospect of a French nuclear physicist having alleged links to al-Qaeda has sent alarm bells ringing in counter-terrorism circles.
Al-Qaeda in the Islamic Maghreb emerged in early 2007, after the Algerian militant group, the Salafist Group for Preaching and Combat (GSPC), aligned itself with Osama Bin Laden's international network.
The group claimed responsibility for a suicide bombing at the French embassy in Mauritania in August which wounded three people.