PARIS, March 4: Thousands of police and railway workers combed the French railway network in a fruitless search for bombs Thursday, as intelligence agents sought to identify the unknown group threatening to set off a series of explosions unless the government pays millions of dollars.

Experts on terrorism were studying the blackmail notes signed with the initials AZF which were sent to the government over recent weeks, as well as the components of a home-made explosive device planted on a railway viaduct in central France.

One theory was that the group, which described itself in one letter as a "pressure group of a terrorist nature linked to a secularist brotherhood," was motivated by political ideology of the far left or the far right, experts said.

They were also looking at possible links with the AZF chemical factory which exploded in September 2001 in the southern city of Toulouse, killing 30 people and injuring more than 1,000 others. The blast, officially described as an accident, created enormous local resentment. -AFP