The authorities said the 18-year-old was connected to the main suspect and had recently sworn allegiance to the Islamic State.

The authorities in Vienna have arrested a third teenager in connection with a foiled terrorist attack on a Taylor Swift concert in the city this week.

They said that they believe that the man, an 18-year-old Iraqi citizen living in Austria who is connected to the main suspect, was not part of the plan but had been in touch with the plotters and had recently sworn allegiance to the Islamic State.

Ms. Swift was scheduled to stage three concerts in Vienna from Thursday through Saturday, but all three performances were canceled after the authorities arrested two teenagers over a plan to attack the sold-out, 50,000-seat stadium. Chancellor Karl Nehammer of Austria said the plot had been designed to leave a “trail of blood.”

Since arresting two other teenagers on Wednesday, the authorities have been racing to investigate the planned attack, although after what the police said was a full confession by the main suspect, they said there was no longer an imminent danger.

The police are looking into a network of people around the main suspect, a 19-year-old Austrian citizen of North Macedonian descent who they said had radicalized himself online and sworn allegiance to the Islamic State.

Citing privacy rules, the authorities have declined to name the suspects publicly, but they said that both teenagers arrested on Wednesday were born in Austria and held Austrian citizenship.

On Friday, the interior minister, Gerhard Karner, announced that he was starting the process of revoking the Iraqi man’s residency permit, under a special provision designed to deal with dangerous refugees or immigrants, because of his recent pledge of allegience to the Islamic State.

The police have said that while he was not part of the plot, the man was in the same “social environment” as the main suspect and that he had contact with him.

During a raid on the main suspect’s house on Wednesday, the police said, officers found chemicals used to make bombs, as well as explosives, timers, machetes, knives and a functioning police siren, which investigators believe he planned to use to gain access to or move around the area around the stadium.

The police are currently in the process of forensically analyzing electronics and other items found in the search of the main suspect’s house.

The concert cancellations affected about 200,000 Taylor Swift fans, some of whom had traveled to Europe from other continents to see her perform as part of her Eras Tour. Ms. Swift has not commented publicly on the cancellations.

A 15-year-old boy who was held for questioning on Wednesday about the plot has been released and is being treated as a witness, the police said. They said that they had determined he was not part of the plot but that he knew many of its details and had helped corroborate some key elements of the main suspect’s confession.