Both suspects in a foiled plot to attack Taylor Swift shows in Vienna appeared to be inspired by the Islamic State group and al-Qaida, Austrian authorities said Thursday, and investigators found bomb-making materials at one of their homes. Officials said one of the two confessed to planning to “kill as many people as possible outside the concert venue.”

Three sold-out concerts were canceled a day earlier because of the plot, devastating Swifties from across the globe. Many of them had dropped thousands of euros (dollars) on travel and lodging in Austria’s expensive capital city to attend the Eras Tour shows at the Ernst Happel Stadium, which sat empty Thursday morning as media filmed outside.

Concert organizers said they stood behind their decision, saying they expected up to 65,000 fans inside the stadium at each concert and as many as 30,000 onlookers outside, where authorities said the suspects planned to strike.

Officials told reporters that the main suspect, a 19-year-old Austrian, began working on his attack plans in July, and just a few weeks ago uploaded to the internet an oath of allegiance to the current leader of the Islamic State group militia. He planned to use knives or homemade explosives.