A French physicist arrested last week while working at the world's largest atom smasher has told investigators he corresponded over the internet with a contact in north Africa's al-Qaida branch, a judicial official said today.

The exchange vaguely discussed plans for terror attacks, but nothing concrete was planned, the French 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 south-eastern French city of Vienne.

The brother was released from custody on Saturday. 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.

US monitors picked up the exchange between the scientist and his contact in the militant group, known as al-Qaida in the Islamic Maghreb. 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 Organisation for Nuclear Research has said. He worked on one of a series of research projects along the 17-mile (27-km) circular tunnel under the Swiss-French border.