According to a report in the Kronen Zeitung newspaper, the Federal Office for the Protection of the Constitution and Counterterrorism requested the increased vigilance due to increased threat of radical-Islamist terrorist attacks, however this has not been officially confirmed.
In the past weeks, at least three suspicious packages triggered evacuations of subway stations.
On September 14th, a discarded suitcase triggered the evacuation of the U1 station at Stephansplatz. The following month, the number 71 tram was suspended due to a suspicious package at Schwarzenbergplatz on October 24. Then on December 18th, the U1 station at Reumannplatz was evacuated, again due to a suspicious package.
The Austrian Press Association (APA) was told by Wiener Linien that the increased vigilance was 'not uncommon.' The spokesman said that when the threat level is raised, staff of the Wiener Linien are asked to be especially vigilant, and to pay extra attention to unclaimed luggage or similar items. The speaker refused to confirm if the alert level was linked to a specific threat of terrorist attacks.