A 21-year-old man pleaded guilty in an Austrian court on Tuesday to charges that he plotted an attack at a Taylor Swift concert in Vienna two years ago, according to the Austria Press Agency, the national newswire.

The defendant, identified by the authorities only as Beran A. to comply with local privacy laws, was arrested in August 2024, shortly before Ms. Swift was set to give the first of three concerts at a stadium in Vienna.

The trial of Mr. A., who was 19 when he was arrested, began on Tuesday in Wiener Neustadt, a city roughly 25 miles south of Vienna. He and another man still face charges in connection with a separate alleged plot.

Here’s what to know about the case.

What happened in 2024?

After being warned by the C.I.A. of a possible plot, the Austrian police raided Mr. A.’s home in Ternitz, south of Vienna, shortly before Ms. Swift’s first concert was scheduled to take place.

Inside his home, which he shared with his parents, the police said they found machetes, knives, explosives, timers and chemicals for making explosives, as well as steroids, Islamic State propaganda and 21,000 euros in counterfeit bills. Mr. A. had made bombs using instructions found online, the police said at the time.

After the raid, Ms. Swift canceled the three concert dates in Vienna, disappointing more than 150,000 fans, many of whom had traveled to Austria from abroad.

What are the accusations?

In February, the Vienna Public Prosecutor’s Office filed charges against Mr. A., accusing him of terrorist acts and belonging to a criminal organization. The prosecutors said he had built a bomb using Islamic State instructions available on the internet and had tried unsuccessfully to procure guns and a grenade. He was also accused of using social media to incite terrorist acts and share the militant group’s propaganda.

Prosecutors said he had planned to target the second of Ms. Swift’s three scheduled concerts. He intended to set off a shrapnel bomb in a crowded space outside the venue, the prosecutors said, and then use other weapons against people in the crowd. Karl Nehammer, Austria’s chancellor at the time, said the intent was to leave “a trail of blood.”

Who is the defendant?

Mr. A. is an Austrian citizen, born in the country to a family from North Macedonia, according to news reports. He appeared to have been radicalized online, the authorities said after his arrest. He quit his job days before the attack, telling colleagues he had “big plans.”

Mr. A., who has been in custody for 630 days, has cooperated with investigators, the authorities said. He confessed to the plot during his first police interrogation after his arrest, investigators said at the time.

Who else is standing trial?

Another 21-year-old man, identified as Arda K., is charged with separately plotting with Mr. A. to carry out attacks in Saudi Arabia, Turkey and the United Arab Emirates, all of which were meant to happen at the same time, according to the Vienna prosecutors. Both men deny participating in a coordinated plot, though Mr. K. has admitted traveling to Turkey before abandoning a planned attack there, according to the Austria Press Agency.

Who else has been arrested?

A third man accused of involvement in that second alleged plot, Hasan E., is currently imprisoned in Saudi Arabia, the news agency reported. Prosecutors said he flew to Mecca and followed through with an attack there, stabbing a security guard in the neck, the agency said.

In a separate case last year, a German court convicted a Syrian teenager living in Germany of helping Mr. A by interpreting Arabic bomb-building instructions and an oath of allegiance to ISIS.

A fifth person, arrested by the police in Vienna in August 2024 in connection with the Taylor Swift case, was released without charges.

How long will the trial last?

The stop-start proceedings will be held over several days, spread out across several weeks. The last hearing is expected at the end of May.

If convicted in both cases, Mr. A faces up to 20 years in prison.