The surroundings were grim and forbidding, a notorious jail run by Afghanistan's feared security service for those taken prisoner in the bloody war with the Taliban.

Among the inmates: Shakirullah Yasin Ali; a small, frail boy, just 14 years old, arrested as he prepared to carry out a suicide bombing against British and American targets. "If I had succeeded, I would be dead now, I realise that," he said in a soft, nervous voice.

"But those who were instructing me said that if I believed in serving God it was my duty to fight against the foreigners. They said God would protect me when the time came."

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It was a suicide bomber like Shakirullah who, on Sunday, claimed the lives of three more British soldiers in Helmand, bringing the total number of UK fatalities in Afghanistan to 100.

The Independent spoke to Shakirullah, a Pakistani Pashtun, one of the youngest ever suicide bombing suspects, after he was captured in a raid at the town of Khost in Afghanistan.

Sitting cross-legged on the floor of the prison run by Afghan intelligence, the NDS, Shakirullah said: "I do not know what is going to happen to me. All we were told was the British and the Americans were in Afghanistan and they were killing Muslims.

"All I know is what the mullahs told me and kept telling me, that the British and the Americans were against God," he said with his head bowed down, his hands twisting a handkerchief.

Shakirullah, one of four children of Noor Ali Khan, a farmer, lived at the village of Tandola in the Pakistani region of South Waziristan. He said his education was at a madrassa run by two imams, Mullah Saleb and Mullah Azizullah. About 50 students between 13 and 22 attended the school, where the syllabus consisted of learning the Koran by heart, interspersed with political lectures.

About two months ago, he finished a first course in Koranic studies. He was then approached by the two mullahs who told him that the time had come for him to serve God in Afghanistan.

"At first, I did not know what I was supposed to be doing, then Mullah Saleb said I would be striking a blow against the foreigners, the British and the Americans, and get justice for all the people being killed. I was told I must leave at once and they would talk to my family on my behalf. I wanted to see my mother and father but I was told that was not possible for security reasons. That upset me but I thought I will be seeing them again as soon as I got back. They said my family would get well paid for what I was doing."

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On the way to Afghanistan Shakirullah said he was told by a mullah that his mission would involve driving a car bomb. "I said I did not know how to drive but they said they would teach me, they said I would not have to drive far. Mullah Saleb said it was too late to stop. He kept saying that to be a good Muslim I must fulfil my duty. I was missing my family but I did not know how to go back to my village and I did not know anyone in the area I could run to. There was nothing I could do except pray I would be all right and my family would be all right."

Shakirullah says he was driven across the border and taken to a house in the city of Khost. "There were a few more people there and the leader was a man they called the Doctor, he and Mullah Saleb took me for driving lessons and took me to sermons in the evening. The Doctor brought the explosives in two bags for the car and he was the one who made the bomb. I was told I would soon be ready to carry out my mission."

However, the car being prepared for the bombing, a Toyota Corolla, had stalled a few times while Shakirullah was being taught to drive and, on one occasion, he and the Doctor had been closely questioned by the police.

Forty-eight hours later, the house where they were staying was raided by Afghan and Nato forces. "I had been told by the mullah that I was ready to go, the time was right. But then they came during the night, the soldiers, and smashed down the doors. There were Afghans and foreigners. A gun was stuck to my face and I thought I was going to be killed. They dragged us all out and took us to a prison."

Shakirullah's attack may have been prevented but not that of the bomber who took the lives of Privates Nathan Cuthbertson, 19, Charles David Murray, 19, and Daniel Gamble, 22. They had been going to speak to local people when a bomber detonated an explosive vest strapped to his chest.

Last night, their families paid tribute to their loved ones. Pte Murray's family said: "David was the best son, brother, grandson, nephew, cousin and friend any of us could hope for. Although his time with us was short, he lived every second to the full and taught us the meaning of life."

The parents of Pte Gamble said: "Dan died doing the job he was so proud to do, with the regiment he was proud to be part of. He was special because he had trained in the Afghan Pashtu language. He was special to his family and friends – a true hero in every sense."

Pte Cuthbertson's company commander, Major Russell Lewis, said he was "a talented, motivated individual. He always had a smile on his face and relished the challenges faced by the professional soldier. "

British victims of the latest suicide attack

Private Daniel Gamble

Daniel Gamble, 22, from Uckfield, East Sussex, joined 2nd Battalion, the Parachute Regiment in 2006 and became his platoon's linguist, working to understand Afghan culture and society. His parents said he was proud of his job and regiment.

Private Charles Murray

Born in Dumfries and brought up in Carlisle, Charles David Murray followed a family tradition when he joined up. His father, grandfather and uncle were all in the forces. Pte Murray, 19, known as David, was serving as a rifleman.

Private Nathan Cuthbertson

The 19-year-old from Sunderland had been fast-tracked through the army since joining aged 16, and within a year of joining 2nd Battalion, the Parachute Regiment, he had passed the infantryassault engineers course, usually reserved for more experienced soldiers. He was his platoon's machine gunner.