Last week was no exception. On Monday night, police in Vienna discovered - and safely exploded - a letter bomb which had been intended for the stepmother of the Interior Minister, Caspar Einem. The device bore all the hallmarks of the more than 20 such bombs that have been dispatched over the past three years, and it only missed its target because it had been sent to an out-of-date address.
The type of explosive used, the nature of the intended victim (the relative of a person seen to be "soft" on foreigners), and the style of the operation all led police to suspect that the latest letter bomb was from the same person. But the real giveaway came in the fragments of the letter that survived the explosion.
They contained a reference to "Count Gerold", an obscure figure from ancient Austrian history who is credited with having repulsed some fierce attacks by marauding Slavs and Avars in the late 8th century. With that, it was clear that the Bajuvarische Befreiungsarmee - or Bavarian Liberation Army (BLA) - had struck again.
"Whoever is behind the letter bombs is obsessed with what he regards as the heroes of Austrian and Germanic history," said Gustav Spann, one of the many Viennese historians who has been pressed into service in a bid to throw more light on the mystery perpetrator. "But his view of history is distorted. The mythologising of a supposedly glorious Teutonic past could come right out of the Nazi time."
The BLA has been cited in nearly all the letters claiming responsibility for the letter bombs and a number of linked pipe bombs which among them have caused four deaths and 11 injuries, but is almost certainly fictitious. Its name is believed to hark back to the Baiuoarii, a Germanic tribe which settled in the region now known as Bavaria in the 6th century, and which then extended along the Danube into much of what is today Austria - displacing Slavs, Avars and anyone else who got in their way.
In addition to "Count Gerold", past letters have included references to "Earl Luitpold", a little-known 10th-century Bavarian nobleman, and a medieval bishop who was killed, together with Luitpold, in a battle against the Hungarians in 907. Nor is the sender just sticking to the ancient Bajuvarians. Letters claiming responsibility for the first wave of bombs in December 1993 all mentioned Count Rudiger von Starhemberg, the 17th-century Austrian nobleman who successfully led the defence of Vienna against the Turks in 1683 and a man long revered in Austrian neo- Nazi circles.
The references to von Starhemberg led police to suspect that the perpetrator (or perpetrators) must belong to the neo-Nazi scene, particularly as their targets were all people known for their foreigner-friendly attitudes. But as the bombings continued and the historical allusions became ever obscurer, they began to fear they were up against something much tougher to crack.
"The technical perfection of the bombs themselves and the formality of the writing style means we are not dealing with your average petrol-bomb- throwing neo-Nazi," said Robert Sturm, the spokesman for the police investigation. "We are up against a very intelligent opponent."
Like the infamous "Unabomber" in the US, Austria's letter bomber enjoys taunting the police when they make mistakes defusing his home-made devices. He also mocks them for pursuing false leads.
Opinion is divided as to whether there is just one person behind the BLA, or a small group. But experts agree that whoever is writing the letters is well-educated, probably aged between 50 and 70, and extremely xenophobic.
"The perpetrator's choice of historical hero makes it clear that he feels threatened by Slavs and people from the east," said Mr Spann. "He wants to put himself in the same tradition as the likes of Gerold and von Starhemberg."
The dismantling of the Iron Curtain in 1989 and the later eruption of war in the former Yugoslavia led to a dramatic increase in the number of foreigners, in particular east Europeans, seeking refuge in Austria. The far-right populist Jorg Haider wasted no time in fanning the flames of xenophobia to further his own political rise.
But while some Austrians would undoubtedly like to see fewer foreigners, there is universal revulsion for the letter bomber. The call to "liberate" ancient Bajuvaria also leaves them cold. "While there may be some sympathy for the aims, there is none whatever for the methods," said Wolfgang Neugebauer, an authority at Vienna's anti-fascist Documentation Centre. "Most people see this letter bomber as a sinister eccentric."
For the police, it has all become extremely embarrassing. Having interviewed more than 50,000 people and offered a reward of 14m schillings (pounds 775,000), they still do not appear to be any closer to solving the case. Some have accused them of not taking crimes committed by right-wing terrorists seriously enough, while others blame sheer incompetence.
In their defence, the Austrian police feel they have one convincing argument: they point out that it took their US counterparts 17 years to track down the "Unabomber".