Vienna has stepped up security around the city following warnings of possible attacks between Christmas and New Year’s Eve. Police are now on alert and are warning citizens to be wary, however they have not specified whether the attacks will be by gun or bombs or if there were specific targets. Despite the threat, police are not canceling specific events.

Names of potential attackers were given to police, but investigation into these names did not yield concrete results.

“We do not know if these people exist in real life, or if they are only names with no real person behind them,” said Christoph Pölzl, a spokesman for Vienna police. “We have no evidence that they are in Vienna, and we have no evidence that they are even in Europe.” Other capitals have received the warning, but Pölzl failed to say which ones.

On their part, Viennese authorities said they would increase surveillance in crowded areas including transport hubs. They plan to pay close attention to suspicious bags or suitcases.