The 32-year-old Frenchman of Algerian origin, who works on the Large Hadron Collider, is suspected of involvement with North African group Al Qaeda in the Islamic Maghreb. He was in France.
A French judicial official says preliminary charges were filed Monday for "criminal association with a terrorist enterprise." The official spoke on condition of anonymity because the investigation is ongoing.
Under French law, filing preliminary charges gives the investigator time to pursue the inquiry before deciding whether to send a suspect for trial or drop the case.
French judicial officials say the suspect has acknowledged that he corresponded online with the group and vaguely discussed plans for terror attacks. In line with French judicial policy, he has not been identified.
The well-educated physicist is one of more than 7,000 scientists working on the Large Hadron Collider, which straddles the border between France and Switzerland.
"This guy has a doctorate in particle physics, so he's clearly an intelligent person. It does take some intelligence, it does take some dedication to achieve qualifications at that level," said James Gillies, spokeman for the European Organization for Nuclear Research, known as CERN, which operates the collider.
Gillies said that security controls to access the office where the suspect worked were fairly light but added that his "card didn't give him access to any of the underground facilities" and that there was nothing that would have interested terrorists.
"There's nothing in there that people can steal and use for terrorist ends, nothing at all. It's all about personal safety. There are areas where we have cryogenic liquids, high magnetic fields, particle beams and so on, where you need specialist knowledge to be able to go there," Gillies said.
The suspect worked in France, at CERN in Geneva and at the Swiss Federal Institute of Technology in Lausanne, Switzerland.
"We are pretty shocked and surprised," said Jerome Grosse, spokesman for the institute, where the suspect worked as an instructor in experimental physics. At CERN, he worked in a laboratory to understand phenomena such as anti-matter and the Big Bang theory on the origin of the universe.
Grosse said that the scientist had not been seen at work for most of the year because he was ill but that he had been in touch with the institute via e-mail.