Anti-terrorist police detained three suspects in connection with an investigation into a mysterious group's threats to bomb French railways, police said Friday.

The suspects, two men and a woman, were taken into custody Thursday in Paris and the suburban Val-de-Marne (search) region, police officials said. They were being held for questioning at the headquarters of French anti-terrorist police.

An obscure group that calls itself AZF (search) has threatened to blow up bombs at French railway targets unless it is paid millions of dollars.

On Thursday, the group issued a cryptic letter suggesting it could carry out an attack to surpass the terror bombings that killed 190 people in Madrid, Spain. But the group, which previously claimed to have mined railway tracks, also announced it was suspending its operations so it can perfect them.

The letter came a day after a bomb was found half-buried on a train track near the town of Troyes (search), some 100 miles southeast of Paris, triggering a massive inspection of France's rail network.

AZF has not carried out attacks, but its threats to blow up rail targets have heightened concerns — laid bare by the March 11 train bombings — about the vulnerability of European public transport systems.

Police said the three suspects' movements corresponded in a "troubling" manner with certain elements of the AZF inquiry, police said. They were questioned all night, police added.

AZF first contacted the government in December, then threatened in February to attack railway targets. The group directed authorities to a bomb, recovered Feb. 21, that was buried in the bed of a railway line near Limoges (search) in central France.

That bomb and the second one found Wednesday were made from an explosive mixture of nitrates and diesel fuel. The second bomb had seven detonators, attached in the same way as the first, and both were housed in identical see-through plastic boxes, police said.

The discovery of both bombs prompted the state train authority to send about 10,000 employees out on foot to check 19,800 miles of track.

Police say they know little about the group. They have communicated with AZF using special phone lines and newspaper classified ads that addressed the blackmailers as "My big wolf." Investigators signed off as "Suzy."

AZF is the name of a chemical factory that exploded in Toulouse (search), in southwest France, in 2001, killing 30 people. Authorities said the explosion was accidental.