The suicide note read: “I am a soldier of 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.”

Jurors were told the boy had also written a note with bullet points including “run down the non-believers with a car” and “strike the infidels who oppose Allah in the neck”.

The defendant, from Rhondda Cynon Taff, denied preparing for an act of terrorism, two charges of encouraging terrorism online and two charges of possessing editions of an Isis propaganda magazine.

He told the jury at Birmingham crown court that he had a “stupid interest in the gory” and was curious about Isis but claimed he had no intention of carrying out a terrorist attack.

In the witness box he said he did not possess a copy of the Koran, was not a Muslim and ate ham.

Insisting he was merely curious, he told the court: “I wanted to see how easy it was for people who had an interest in terrorism to go online and get information because the police and the government are trying to crack down on terrorism and radicalisation. I wanted to see if it was possible, not for me but from someone else’s point of view.”

His defence counsel, Delroy Henry, likened the youth’s interest in Isis to rubberneckers on a motorway. “He was drawn into it, curiosity getting the better of him,” Henry said.

Police raided the boy’s home on June 30, 2017. He surrendered his phone and told police his Instagram password was “TruckAttack”. Officers carried out a “safety interview”, which allows questions to be asked without a lawyer being present.

The boy said he had been talking to someone on Instagram calling themselves Al-Baghdadi for about a week. That person had told him he would go to hell because he did not believe in Islam and he needed to perform an act of terrorism if he wanted to go to paradise. The jury was told that the leader of Isis is Abu Bakr al-Baghdadi.

The defendant said: “They started telling me about, instead of living your life like a good Muslim … basically kill people who don’t believe in Islam, because one day there will be more Muslims than Christians and Muslims, Islam will dominate the world.”

He accepted in this interview that he had posted material on Instagram about an attack on Cardiff but claimed he had not meant any harm. He also conceded he had put the hammer and knife in his bag and had taken it to school that day, but he claimed to the police that he had not intended to use them to attack anybody.

The boy said: “I never thought about actually doing it. Even though I did carry a hammer and a knife I never thought about doing it for one minute.”

In the days before the Bieber concert at the Principality stadium, he searched for information about the event. The following day he searched for “Justin Bieber security”. Other search terms included “how to steal a car” and “how to steal a car from a non-believer”, and he also visited a webpage describing how to kill someone with a knife.

The jury was not told how the teenager was caught. But outside court, DS Lee Porter, of the Welsh extremism counter terrorism unit, suggested friends may have contacted the authorities.

“This case has highlighted the ongoing concerns with young people gaining access to extremist material on the internet and how quickly that can lead to radicalisation. This individual actively sought out that material,” he said.

“As he became radicalised his behaviour became a concern. Friends were aware that he was displaying extreme views and he had possession of a knife in school.”