A 17-year-old British boy has been found guilty of plotting a terror attack in Cardiff, Wales, the same day as a Justin Bieber concert last June.

The Birmingham Crown Court found the teenager, who cannot be named, guilty of five terror-related charges -- one for engaging in the preparation of a terrorist act, two for encouraging terrorism, and another two for possessing terrorist information.

According to BBC, the boy was an A-level student from Rhondda Cynon Taff. He was arrested on the day of Justin Bieber's concert at the Principality Stadium on June 30, carrying a claw hammer and a gutting knife on his rucksack.

During the trial, the court discovered a "martyrdom letter," which revealed the defendant's plans for the supposed attack.

In the suicide note, he wrote "I am a soldier of the Islamic State. I have attacked Cardiff today because your government keep on bombing targets in Syria and Iraq. There will be more attacks in the future."

Further investigation revealed that the teenage boy created an Instagram account for the sole purpose of encouraging jihad and supporting the extremist al-Qaeda. The password for the account was "TruckAttack."

According to The Guardian, the boy revealed in a safety interview, where no lawyer was present, that he had been talking to a so-called Al-Baghdadi via Instagram for about a week before the attack. He claimed that the man told him he would go to hell for not believing in Islam, and the only way to redeem himself and go to paradise was to perform an act of terrorism.

Police, however, say that based on his internet searches, the boy's interest in Isis started a year before, when he began searching for an Islamist fighter known as Jihadi John. Other searches in his history include: "Justin Bieber security," "how to steal a car," "ISIS beheading video," and other terrorist-related terms.

His barrister, Delroy Henry, defends that the boy was not planning an attack. He just had a "stupid interest in the gory." The teenager also said "I never thought about actually doing it."

His sentence will be revealed on Jan. 10.