A mysterious group that claimed to have planted bombs on the French railroad network announced Thursday it is suspending its terror threats while it improves its ability to carry them out.

In two letters, addressed to President Jacques Chirac (search) and Interior Minister Nicolas Sarkozy (search), the group, which calls itself AZF (search), said that "there are currently no bombs capable of functioning on the French rail network."

"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 one-page typed letter said.

The Interior Ministry said it received the letter Thursday morning, and made a copy available to the press. It carried the letters "AZF" and an arrow in the top left corner.

The letter came a day after a bomb with seven detonators was found half-buried on a passenger train track near the town of Troyes (search), southeast of Paris, triggering a massive inspection of the tracks.

It was the second bomb discovered hidden under tracks in just over a month — and the second inspection of thousands of miles of track.

The discoveries came in the wake of the March 11 train bombings in Madrid, Spain, which prompted nations around the world — including France, Greece, Poland and the United States — to heighten security on train lines. The Spain bombings killed 190 people.

In the United States, Amtrak increased police patrols and intensified electronic surveillance of bridges and tunnels. Major cities, including New York and Washington, also have boosted security on their subway systems.

In Amsterdam, Netherlands, on Thursday, police evacuated hundreds of passengers from the central train station because of a bomb threat. Traffic also was stopped at railway stations in at least three other cities.

The Amsterdam station was reopened more than an hour later after police determined the threat was "a false alarm," a spokesman said.

France has been on high alert since the Madrid attacks, with soldiers patrolling trains, train stations and airports. Suspicion for the Spain attacks has focused on an alleged Morocco-based terrorist cell believed linked to Usama bin Laden's Al Qaeda network and on Al Qaeda itself.

The French government first revealed the threat from AZF on March 3, when it said a powerful bomb was found Feb. 21 on the Paris-Toulouse train line near Limoges in central France. The government said investigators were told of the bomb's location by the previously unknown group.

The government said AZF was threatening to detonate nine bombs planted in the rail network unless it was paid sums totaling $5.2 million. The group increased the ransom demand to $6.4 million in a March 8 letter to Chirac, police said.

In the new letter, AZF also included a cryptic threat, saying that if the money were not paid when asked, "France will surpass without glory the sad Spanish records," an apparent reference to the Madrid bombings.

"So understand well that we in no way renounce obtaining the sum ... that you should consider as a subsidy," the letter said.

The first bomb, found in February, was claimed by AZF. The letter did not say whether it planted the second device discovered Wednesday by an employee of the state-run train authority about 105 miles southeast of Paris.

The bomb, made of a mixture of nitrates and diesel fuel, was contained in a clear plastic box measuring about 8 inches by 8 inches, the Interior Ministry said.

The bomb was attached to a battery linked to the detonators and a handmade timing device, the ministry said.

The state train authority launched a massive search of the rail network Wednesday evening, with about 10,000 employees checking 19,800 miles of track on foot. A similar search earlier this month found no signs of bombs or foul play.

Investigators have carried on a cat-and-mouse game with AZF, using special phone lines and newspaper classified ads addressing the group with code names like "My big wolf." The investigators sign off as "Suzy."

The daily newspaper Liberation said Thursday that police placed more ads March 12 and 17 that said "discretion assured" and included phone and fax numbers.

Prime Minister Jean-Pierre Raffarin (search) has said there had been "several contacts" with AZF. State-run France-2 TV reported Wednesday a rendezvous had been planned for last weekend but AZF apparently did not follow up.

In the letter, AZF described itself as "a small brotherhood" with no grievances against the French government.

"Our true objective is to strike a decisive blow against the depraved spirit that prevails today in most human actions," the letter said.

AZF is not the only mysterious group issuing threats.

Letters signed by the "Servants of Allah, the Powerful and Wise," addressed to Raffarin, were received last week by two newspapers.

The letters threatened possible terror attacks against France and French interests to punish the country for a new law banning Islamic headscarves in public schools starting in September.

French embassies in Muslim countries in Africa, the Middle East and Asia received the same letter.