Taylor Swift canceled three concerts in Vienna after Austrian authorities arrested two people, accusing them of planning attacks on large-scale gatherings – including the singer’s shows – in the European capital. The arrests come amid a string of similar terrorist threats targeting major events in Europe this summer.

One of the suspects is a 19-year-old who allegedly pledged allegiance to the Islamic State, authorities said. That suspect, an Austrian citizen, was arrested Wednesday in the town of Ternitz.

“We have identified relevant preparatory actions and also that a focus of the 19-year-old was the Taylor Swift concerts in Vienna,” Franz Ruf, the public security director at Austria’s interior ministry, said in a joint news conference with Police Chief Gerhard Purstl on Wednesday. No information was shared about the second suspect.

Taylor Swift performs at San Siro stadium in Milan, Italy, for her Eras Tour concert on July 13. Claudio Furlan/LaPresse via AP

Swift’s concerts were scheduled for Thursday, Friday and Saturday at Ernst Happel Stadium and were expected to draw more than 150,000 people across all three nights.

Ruf said the arrested people were radicalized online, and that officers found chemical substances after searching the first suspect’s home. Investigators are working to determine if those substances could have been used to construct a bomb, according to authorities.

Though Purstl said that security would be increased for the concerts, including police dog units and anti-terror teams, the official Taylor Nation Instagram account indicated later on Wednesday that all three shows were canceled.

“With confirmation from government officials of a planned terrorist attack at Ernst Happel Stadium, we have no choice but to cancel the three scheduled shows for everyone’s safety,” reads a post by Austrian concert promoter Barracuda Music, which Taylor Nation reposted. Barracuda Music and Swift’s official website said that ticket-holders can expect to receive full refunds within 10 business days.

Swift’s publicist did not immediately respond to a request for comment.

Swift’s Eras Tour is the highest-grossing concert tour of all time, and fans have traveled from all over the world to see the singer perform at her European shows. As her remaining U.S. shows boast individual ticket prices over $1,000, some American fans have gone as far as to plan entire summer vacations around Swift’s European tour dates. Taken together, airfare, hotels and a concert ticket in Europe can sometimes cost less than a seat at a show in the United States.

Among those Americans who were set to see Swift abroad are Hunter Cooke and his wife, who had booked a flight to Vienna from Austin. The suspected terror plot derailed those plans, Cooke told The Washington Post.

Cooke and his wife were particularly concerned about reports they’d seen in Austrian media outlets about three suspected participants in the plot apparently still at large. “Even if they didn’t cancel [the shows], there was a genuine question we had of whether we want to go to this,” he said.

The Post could not independently confirm the local media reporting.

Cooke, 33, said he and his wife are planning to adopt a child and had planned this vacation as their last “big trip” before moving forward with the adoption process. Prospective parents can receive a foster placement at any time, which makes travel difficult, Cooke explained from the airport. His wife was searching for other flights, in hopes that the couple could still “make a vacation of it,” even if not in Europe.

“We’re out $3,000, and hopefully we’ll get reimbursed, but if that’s the price for not going on a five-day trip where we’re fearing for our lives, then that’s what it is,” Cooke said. “Obviously, it’s not worth dying to see Taylor Swift.”

Austrian Chancellor Karl Nehammer, in a tweet Wednesday, called the cancellation a “bitter disappointment” for Austrian fans but thanked Austria’s emergency services, police forces and foreign intelligence for preventing a “tragedy” and “investigating at full speed.”

At the end of a 2017 Ariana Grande concert in Manchester, England, a suicide bombing killed 22 people and injured over 100 more. An official inquiry last year found that MI5, Britain’s domestic intelligence agency, could have prevented the attack but failed to act quickly enough and missed critical information, the Associated Press reported.

Terror threats have disrupted numerous major events across Europe this year. In March, a grievous attack on Crocus City Hall in Moscow killed at least 137 people and hospitalized over 100 others. The Islamic State claimed credit for the attack, and intel assessments largely agreed that Islamic State-Khorasan, the Afghanistan and Pakistan arm of the group, was responsible.

In April, local police increased security for the Eurovision Song Contest in Malmo, Sweden, amid heightened concern over possible attacks. In May, French officials said an 18-year-old man had been arrested for allegedly planning an attack on the Olympics in the name of the Islamic State, Reuters reported. In May, New York state and local police officers said weeks of threats against the Cricket World Cup were tied to ISIS-K, and tightened security as the June India-Pakistan match was reportedly the focus of threats posted to a pro-Islamic State online platform.

Intelligence officials in the United States have warned since at least March that the war in Gaza and fury at U.S. support for Israel could galvanize existing terrorist groups and inspire fresh threats.

Washington Post writer Kate Brady contributed to this report.