In the fight against the international terrorist threat in Indonesia, one man has become an invaluable ally. Nasir Abbas explains why, after men he trained carried out the Bali bombing in 2002, he decided to change sides.
Nasir Abbas is also in demand from intelligence agencies overseas
For many years Nasir Abbas was one of the most wanted jihadis in South East Asia.
He was a member of al-Qaeda's regional affiliate, Jemaah Islamiya (JI).
The Malaysian trained the Bali bombers in Afghanistan, established a jihadi training camp - Camp Hudabiya - in the dense jungles of Mindanao in the southern Philippines, and rose to become the head of JI's military training division, known as Mantiki Three.
He was close to some of the most notorious militants in the region and brother-in-law of Mukhlas, the mastermind of the 2002 Bali bombings.
Those he trained and those he knew went on to operate not just in South East Asia, but in other parts of the world.
Mr Abbas seems an unlikely jihadi.
He is quietly spoken, well-mannered and seemingly gentle, belying the fearsome reputation acquired over the years.
I asked Indonesia's top anti-terrorist police officer, General Ansyaad Mbai, how he used to regard his adversary.
"He was a very dangerous man because he was one of the key figures in JI," he said.
Mr Abbas makes no secret of what he taught at Camp Hudabiya, which evolved from a clearing in the jungle to become, in his words, a fully-fledged military academy.
"It included individual combat training, small arms and weapons training and basic knowledge about explosives," he said.
Did he train people to kill?
His reply gave a hint of why he eventually underwent such a remarkable transformation.
"I train people for war, for battle," he said. "We are killing for defence. We are fighting for our right. And we are not attacking civilians but soldiers."
According to Mr Abbas' philosophy of jihad, it is acceptable to fight and kill foreign forces occupying Muslim countries like the Soviets in Afghanistan, the Americans in Iraq or the Philippine army occupying ancestral Muslim lands in Mindanao, but killing innocent civilians - men, women and children - is forbidden.
This is the philosophy of modern violent jihad outlined by Palestinian Abdullah Azzam, acknowledged to be the "father" of modern violent jihad.
With this distinction in mind, the 2002 Bali bombings in which 202 civilians died, made Mr Abbas think again about the organisation to which he had belonged for almost a decade.
When he discovered that his former students, whom he had trained in Afghanistan in the early 1990s, were responsible, he was deeply shocked.
"I feel sorry, I feel sin," he said, "because they used the knowledge to kill civilians, to kill innocent people."
It was only when he was arrested six months later in April 2003, that Mr Abbas finally decided to put his past behind him.
As he was taken off for interrogation, he feared the worst.
"I believed that the police were very cruel and used torture to get their answers," he said.
His ultimate test of allegiance came when he gave evidence in court against the spiritual leader of Jemaah Islamiya
But Mr Abbas was in for a surprise. He was treated with civility and Muslim respect.
He was also surprised that so much was known about him and was puzzled as to how his interrogators knew.
He was arrested one evening and kept silent until the following morning.
Then he decided to talk and help the police, because he thought it was God's will.
He said he felt "responsible, in front of God, to stop all these bad deeds."
From that point on, Mr Abbas tried to persuade his former comrades that their interpretation of the Koran was wrong.
He urges them to "return to the right path of Islamic teaching."
But he did much more than that.
He actively assisted the police in tracking down and arresting some of his former comrades and felt no guilt in doing so.
His ultimate test of allegiance came almost two years after his arrest when he gave evidence in court against the alleged spiritual leader of JI, Abu Bakar Ba'asyir.
Mr Ba'asyir was charged with conspiracy in connection with the 2002 Bali bombing.
There was a near riot in court when Mr Abbas gave evidence that Ba'asyir had personally made him the leader of Mantiki Three and had attended a passing out parade of Mr Abbas' graduates at Camp Hudabiya.
Ba'asyir was given a two-and-half year prison sentence.
Mr Abbas continues his work today.
Last week another JI member against whom he gave evidence, Mohamed Cholily, was sentenced to 18 years for involvement in the 2005 Bali bombing.
And earlier this year he provided police with information that helped them track down Azahari Bin Husin, JI's master bomb maker, who made the 2002 Bali bombs.
The jihadi who turned has every intention of carrying on.
The problem for the police is keeping him alive as he has made so many enemies.
JI's Noordin Top, now the most wanted man in Indonesia, has made no secret of what he would do to Mr Abbas should he ever get his hands on him.
Mr Abbas believes God will protect him.
Al-Qaeda: Turning the Terrorists was broadcast on Wednesday, 13 September, 2006 at 2100 BST on BBC Two.