PARIS -- An obscure group trying to blackmail France for millions of dollars threatened an attack to surpass the Madrid terror bombings in a cryptic letter Thursday. But the group, which previously claimed to have mined railway tracks, also announced a suspension of its operations, saying it wants to perfect them. The letter came a day after a bomb was found half-buried on a train track near the town of Troyes, 105 miles southeast of Paris, triggering a massive inspection of France's rail network. For reasons investigators have yet to fathom, the group calls itself AZF. While it has not carried out attacks, its threats to blow up rail targets have heightened concerns -- laid bare by the Madrid train bombings -- about the vulnerability of European public transport systems.
The new single-page typed message, which carried the letters "AZF" and an arrow pointing left to right in the top left hand corner, was received Thursday by President Jacques Chirac's office and the Interior Ministry.
In the letter, AZF suggested bombs it earlier claimed to have laid under rail tracks have been neutralized while it regroups.
"There are today no longer bombs capable of functioning on the French rail network," said the letter, which the Interior Ministry made public. "With the experience gained these last weeks and now conscious of its technological, logistic and other weaknesses, AZF suspends its action for the time needed to remedy this."
The group said it remains determined to extract ransom from the government at some future date. It has demanded $6.4 million.
"When we demand it, pay up with alacrity and let us speak no more about it. Otherwise, France will surpass without glory the sad Spanish records," said the letter, which was laced with cryptic turns of phrase.
Police said they regarded the Spanish reference to be to the Madrid train bombings on March 11, which killed 190 people and were claimed in the name of al-Qaida.
Those attacks prompted heightened security in Europe. On Thursday, hundreds of passengers were evacuated from Amsterdam's central train station because of a bomb threat police later declared a false alarm.
AZF first contacted the government in December and 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 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.
After each bomb discovery, the state train authority sent 10,000 employees out on foot to inspect 19,800 miles of track. The latest search started Wednesday evening. The previous check in early March found no signs of bombs or foul play.
Government spokesman Jean-Francois Cope called for calm, "because it would be a gift for terrorists of all types if we gave the feeling that we fear them."
Police 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."
But still, police say they know little about the group.
The initials AZF are those of a chemical factory that exploded in southwestern France in 2001. But investigators believe that explosion, which killed 30 people, was accidental and it is not known whether it has any link to the railway threats.
Nor do police know how many people they are dealing with. Police say they have heard the voices of a man and a woman, both speaking in French, in telephone contacts with the group.
In its latest letter, AZF described itself as a "small brotherhood" and said its members bear no grudge against the government.
"Our true objective is to strike a decisive blow against the depraved spirit that prevails today in most human actions," the letter said.
Police contacts with AZF appear to have been intermittent. A woman contacted police March 12, after investigators placed a newspaper ad with phone and fax numbers and saying "discretion assured."
The woman asked that a helicopter be readied to carry ransom money in postal sacks, but poor weather and nightfall foiled a delivery, police officials said.
Police placed another newspaper ad on March 17 but got no response.
AZF is not the only mysterious group issuing threats to France.
French embassies in Muslim countries in Africa, the Middle East and Asia received letters, sent March 15 from a post office near the Louvre Museum in Paris, that threatened attacks to punish France for a new law banning Islamic headscarves in public schools.
France's Foreign Ministry said Thursday the letters were being examined.