The threat was contained in an open letter to President Jacques Chirac and couched in exceptionally strong language. After referring to past terrorist attacks and threatening to "destroy France", the letter said: "We are capable of doing much more than this, because we believe that when we cut off your heads, dissect your bodies and scatter you, we are performing an act of devotion that brings us nearer to God." The letter also spoke of "murders and massacres" and said: "You know that we do what we say; recent events prove it."
Written in Arabic over two pages, the letter purported to come from the Armed Islamic Groups (GIA) - the organisation held responsible for the 1995 bomb attacks in France and believed also to be behind the bomb at Port Royal station in Paris on 3 December. Although it contained no specific admission of responsibility for that bomb, it referred to the hijack of the Air France airbus two years ago, and to terrorist attacks "that cost the lives of dozens of your compatriots".
It was signed with the name of Antar Zouabri, "emir of the GIA". Zouabri was recently named in a newsletter regarded as close to the GIA as the organisation's new leader.
The letter apparently arrived at the Elysee Palace last weekend, but first details were released by the authorities, perhaps deliberately, only on Christmas Eve when people were preoccupied with domestic preparations and the media were in good-news mode.
Reports were low key and were accompanied by an interior ministry statement insisting that "all measures of vigilance and security have been renewed and strengthened" and that the government would "not to give in to blackmail, fear or violence". There were also suggestions that the letter was a forgery or a trick. There are many signs, however, that it is regarded by the authorities as genuine.
It now appears, for instance, that although the French government frequently professes ignorance of the GIA's motives for any attacks, it has received similar letters and demands in the past, including before the Air France hijack two years ago and after the abduction of seven French monks from their monastery in Algeria last April.
In the first case, the French authorities refused to negotiate, stormed the plane and killed the hijackers, leaving the GIA threatening vengeance. In the second, the authorities apparently tried to deal with the kidnappers (though they denied it at the time), and the subsequent murder of the monks seems to have resulted from the deal going wrong.
Both times, the attackers demanded the freedom of Abdelhaq Layada, who headed the GIA until June 1993 when he was arrested in Morocco and extradited to his native Algeria where he faces the death penalty. The latest letter also demands freedom for an as yet unnamed group of "brethren".