A French physicist arrested last week while working at the world's largest atom smasher has acknowledged to investigators that he corresponded over the Internet with a contact in North Africa's Al Qaeda branch, a judicial official said Sunday.

The Internet exchange vaguely discussed plans for terror attacks, but nothing concrete was planned, the French judicial official said, speaking on condition that his name not be used because the investigation is ongoing.

The 32-year-old Frenchman of Algerian origin was one of more than 7,000 scientists working on the Large Hadron Collider in Switzerland. He and his brother were taken into custody Thursday in southeastern French city of Vienne.

The brother was released from custody Saturday, the official said.

The physicist was still being held in the Paris area on Sunday, with no charges filed against him. Under French law, terror suspects can be held without charges for up to four days.

U.S. monitors picked up the Internet exchange between the scientist and his contact in the militant group, known as Al Qaeda in the Islamic Maghreb, the judicial official said. The North African group regularly targets government and security forces in Algeria, and occasionally attacks foreigners.

At work, the physicist had no contact with anything that could be used for terrorism, the European Organization for Nuclear Research has said. The experiment where he worked is one of a series of research projects along the 17-mile circular tunnel under the Swiss-French border.

The arrest has added to the woes of the $10 billion particle collider.

The collider started up spectacularly in September 2008 with beams of particles flying in both directions on the first day of trying. But nine days later, a massive electric failure related to a construction fault caused the entire machine to shut down.

It has been undergoing repairs almost ever since with the bill expected to total about $40 million over the course of several years.