A series of letter-bomb incidents in Austria has officials concerned about the spread of ultra-rightist violence across Western Europe.

So far four people have been injured in separate letter-bomb attacks, including Vienna Mayor Helmut Zilk. All the victims apparently were targeted because they were advocates of rights for foreign refugees, Austrian officials say. In addition to the four bombs that exploded, police have intercepted and defused six other devices.

Austrian authorities believe neo-Nazis are responsible for the terror campaign, which began last Friday. They note that some of the bombs carried a written reference to Count Ernst Rudiger von Starhemberg, an Austrian national hero who led the successful defense of Vienna against Ottoman Turkish invaders in 1683.

Unlike some of its neighbors - including Germany and Italy, which have experienced everything from left- and right-wing terrorism to Mafia-inspired attacks in past decades - Austria has been generally free of political violence since the end of World War II.

Austrian authorities on Tuesday raided two homes in search of suspects, saying they were closing in on the bombers thanks to tips provided by concerned citizens. But even if the culprits are caught, it may not mean the end of neo-Nazi violence in Austria.

``If they can make letter bombs, which can be quite complicated, they can easily make other devices, such as car bombs,'' Wolfgang Neugebauer, a specialist on Austrian right-wing movements, told the Reuters news agency.

The Austrian attacks have prompted some officials in Germany to call for heightened security measures against possible neo-Nazi attacks there. Last week, an underground neo-Nazi magazine called Der Einblick, or Insight, published a ``hit-list'' of 250 German left-wing activists.

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