A third teenager has been arrested in connection with a foiled attack on now-cancelled Taylor Swift concerts in Vienna, Austria's interior minister said Friday.

Austrian Interior Minister Gerhard Karner said an 18-year-old was taken into custody Thursday evening in Vienna after allegedly being in contact with the main suspect. Karner announced the arrest during an unrelated news conference Friday.

Investigators are scrutinizing the "networks" of the suspects, the Austrian interior ministry told The Associated Press on Friday, adding in a statement that investigators have turned to evaluating physical and electronic evidence.

The 19-year-old main suspect and a 17-year-old were arrested Tuesday, while a 15-year-old was also interrogated but was not arrested. None of their names were released, in line with Austrian privacy rules.

Officials said Thursday afternoon that no additional suspects were being sought. They did not immediately offer further details Friday.

Authorities say the plot appeared to have been inspired by the Islamic State group and al-Qaeda. Investigators found bomb-making materials at one of the suspects' homes. Officials say one suspect has confessed to planning to "kill as many people as possible outside the concert venue."

WATCH | Suspect planned to attack fans with explosives, knives, police say: 

ISIS-inspired suspect confesses to foiled Taylor Swift concert attack

Austrian police say a 19-year-old suspect confessed to plotting an ISIS-inspired suicide attack outside a Taylor Swift concert in Vienna. They say he planned to attack fans with explosives and knives.

Officials say the main suspect and the 18-year-old arrested Friday both pledged "oaths of allegiance" to the Islamic State group.

The 18-year-old "comes from the social environment" of the main suspect, Karner said.

"He had been in contact with the main perpetrator, but is not directly connected to the attack plans," Karner said. "But, as was found out a few days ago, he took an oath of allegiance specifically to the IS on Aug. 6."

Foreign agencies aided investigation

Three sold-out concerts were cancelled Wednesday because of the plot, devastating Swifties from across the globe. Many of them had dropped thousands of euros on travel and lodging in Austria's expensive capital city to attend the Eras Tour shows at the Ernst Happel Stadium.

Concert organizers in Austria said they had 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. The foiled attack was planned for Thursday or Friday, according to Karner.

Karner said that Austria's intelligence service had worked closely together with foreign intelligence services to capture the two teens. He did not identify the agencies, but said the help was needed because Austrian investigators, unlike some foreign services, can't legally monitor text messages.

The CIA declined to say Thursday whether U.S. intelligence agencies played any role in the investigation. The U.S. State Department and the broader U.S. government have been in contact with Austrian officials about the alleged plot, State Department spokesperson Matthew Miller said.

The main suspect confessed that he had started planning the attack in July, authorities said. The 19-year-old just a few weeks ago uploaded to the internet an oath of allegiance to the current leader of the Islamic State group.

He was "clearly radicalized in the direction of the Islamic State and thinks it is right to kill infidels," said Omar Haijawi-Pirchner, head of the Directorate of State Security and Intelligence.

Haijawi-Pirchner added that the suspect "wanted to carry out an attack in the area outside the stadium, killing as many people as possible using the knives or even using the explosive devices he had made."

During a raid of the suspect's home in Ternitz, south of Vienna, investigators found chemical substances and technical devices that indicated "concrete preparatory acts," said Franz Ruf, director general for public security at the Ministry of the Interior.

Authorities said they also found Islamic State group and al-Qaeda material at the home of the 17-year-old suspect. That suspect, who has so far refused to talk, was employed a few days ago by a company providing unspecified services at the venue for the concerts.

Austrian chancellor defends cancellation

Austrian Chancellor Karl Nehammer defended the decision to cancel the concerts, saying the arrests of the suspects took place too close to the shows, scheduled for Thursday, Friday and Saturday.

"I understand very well that those who wanted to experience the concert live are very sad," Nehammer told a news conference Thursday. "Moms and dads are looking after their daughters and sons, who were full of enthusiasm and anticipation for this concert. But it's also important that in such serious moments as now, it's inevitable that safety comes first."

Concert organizer Barracuda Music said in an Instagram post late Wednesday that it had "no choice but to cancel the three scheduled shows for everyone's safety."

Barracuda said all tickets would be refunded. The same message was posted under the Vienna dates on Swift's official website.

London shows to go ahead

Swift is also scheduled to perform at London's Wembley stadium in five concerts between Aug. 15 and 20 to close the European leg of her record-setting Eras Tour.

London Mayor Sadiq Khan said that while he understood Vienna's reasons for cancelling, "We're going to carry on." Khan said the capital's authorities were prepared for shows there following lessons learned from a 2017 attack by a suicide bomber at an Ariana Grande concert in Manchester, England, that killed 22 people.

Last month, an attacker in England killed three girls and wounded 10 people in a knife attack during a Taylor Swift-themed dance and yoga class. Swift at the time said she was "completely in shock" over the violence.

Swift has not spoken publicly about the plot or cancelled shows. "Taylor Nation," a verified Instagram page widely believed to be run by her team, reposted the announcement from Barracuda Music in a "story," which is only visible for 24 hours. Her main account has not posted anything.

Shiraz Maher, an expert on Islamic extremism with the Department of War Studies at King's College London, said in a statement to the AP that attackers "prioritize casualties and therefore choose soft targets where they know large numbers of people will be congregating."