A shadowy group that threatened to bomb the French rail network if not paid a large ransom has suspended its threats, French officials say.
SNCF has increased security since the group sent blackmail demands
But the group, which calls itself AZF, warned it could return with attacks worse than the Madrid train bombings, in a letter to the interior ministry.
"Pay up... or else France will surpass ingloriously the sad records of Spain," the AZF statement read.
It followed Wednesday's bomb find on a track between Paris and Switzerland.
A bomb disposal squad made the device safe.
The authorities said they did not know whether that incident was linked to AZF.
'Force of persuasion'
In a letter addressed to the French presidency and the interior ministry, the group said it was suspending its planned campaign because of technical difficulties.
But the group said that once it had overcome these it would return with a more effective "force of persuasion".
"Understand that we have absolutely no intention of giving up our aim of obtaining the money we need," the letter read, before ending with the words "without bitterness and till soon".
The previously unknown group sent notes to the French president and the interior ministry demanding $4m and 1m euros (£2.5m in total).
In February the group tipped off police to a device it had hidden on a viaduct near the town of Limoges in central France.
The threats prompted a search of France's entire rail network two weeks ago, which failed to find any traces of an explosive device.
After Wednesday's discovery, the state-run rail company SNCF ordered another search of the country's 32,000km (19,800 miles) network.
Earlier on Thursday officials said there had been renewed contact
with the group via Liberation newspaper, reported French news agency AFP.
The interior ministry reportedly placed two messages in the personal column in mid-March, posing as an art dealer wishing to buy works by the sculptress Camille Claudel.
The first message prompted a telephone call the same day from a female group member, with instructions for a ransom drop at an airfield near Paris.
After she was told the weather conditions were not right she said she would call back at a later date.
Since then the ministry has received another threatening letter
but no more telephone calls, officials said.