Austria's leaders unanimously condemned the blast, which happened in the early hours of Sunday morning, some describing it as part of a concerted attempt to destabilise the country.
But even as they spoke, police announced that a 30-year-old man had been injured in another explosion, this time in a small village 65 miles outside Vienna in which many Croatian refugees live. Terezija Stoisits, an MP and supporter of minority rights, warned all foreigners in Austria to be "extremely vigilant" over the coming days.
The Interior Ministry also urged special caution, particularly in the Burgenland province, where both explosions occurred.
If confirmed to have been racially motivated, Sunday morning's explosion in the town of Oberwart would be the most serious extremist attack on foreigners in Austria for at least 20 years.
The four Gypsies who died in the explosion were aged between 18 and 40. They were killed while trying to dismantle a roadside sign on which the words "Gypsies, go back to India" had been scrawled.
According to police, the sign was booby-trapped: explosives had been attached and went off as soon as someone attempted to remove it. Chancellor Franz Vranitzky said Austrians felt "horror and disgust at this cowardly attack perpetrated on members of theGypsy minority". Erhard Busek, the deputy chancellor, warned that its perpetrators were seeking to destabilise Austria and harm the country's international standing. The killings were also condemned by Jorg Haider, leader of the far-right Freedom Party, who is blamed by many for fanning anti-foreigner sentiment in Austria.
While Sunday's blast clearly appeared to have been aimed against Austria's 40,000-strong Gypsy community, yesterday's explosion was attributed to strong resentment against the country's many immigrants, in this case Croats who fled from the war against Serbia.
According to police, a bomb was placed in a dustbin in a central area in the village of Stinatz, home to a substantial Croat minority. A 30-year-old Austrian dustman suffered a serious hand injury when the device exploded.
The proximity of the two blasts sparked fears of a new wave of attacks similar to that which swept the country in December 1993.
At that time, some 10 letter-bombs were sent to people known for their pro-foreigner sympathies. Five people were injured, including the then mayor of Vienna, Helmut Zilk.
Police yesterday confirmed they were exploring possible links with the 1993 letter-bomb campaign and other racially-motivated incidents over the past year.
Last week the Justice Ministry announced that two neo-Nazis were going to stand trial in connection with the letter-bomb attacks.Reuse content