Khaled looked at me with a broad smile. He was almost laughing. At one point, when I told him that he should abandon all thoughts of being a suicide bomber – that he could influence more people in this world by becoming a journalist – he put his head back and shot me a grin, world-weary for a man in his teens. "You have your mission," he said. "And I have mine." His sisters looked at him in awe. He was their hero, their amanuensis and their teacher, their representative and their soon-to-be-martyred brother. Yes, he was handsome, young – just 18 – he was dressed in a black Giorgio Armani T-shirt, a small, carefully trimmed Spanish conquistador's beard, gelled hair. And he was ready to immolate himself.

A sinister surprise. I had travelled to Khaled's home to speak to his mother. I had already written about his brother Hassan and wanted to introduce a Canadian journalist colleague, Nelofer Pazira, to the family. When Khaled walked on to the porch of the house, Nelofer and I both realised – at the same moment – that he was next, the next to die, the next "martyr". It was his smile. I've come across these young men before, but never one who so obviously declared his calling.

His family sat around us on the porch of their home above the Lebanese city of Sidon, the sitting room adorned with coloured photographs of Hassan, already gone to the paradise – so they assured me – for which Khaled clearly thought he was destined. Hassan had driven his explosives-laden car into an American military convoy at Tal Afar in north-western Iraq, his body (or what was left of it) buried "in situ" – or so his mother was informed.

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It's easy to find the families of the newly dead in Lebanon. Their names are read from the minarets of Sidon's mosques (most are Palestinian) and in Tripoli, in northern Lebanon, the Sunni "Tawhid" movement boasts "hundreds" of suiciders among its supporters. Every night, the population of Lebanon watches the brutal war in Iraq on television. "It's difficult to reach 'Palestine' these days," Khaled's uncle informed me. "Iraq is easier."

Too true. No one doubts that the road to Baghdad – or Tal Afar or Fallujah or Mosul – lies through Syria, and that the movement of suicide bombers from the Mediterranean coasts to the deserts of Iraq is a planned if not particularly sophisticated affair. What is astonishing – what is not mentioned by the Americans or the Iraqi "government" or the British authorities or indeed by many journalists – is the sheer scale of the suicide campaign, the vast numbers of young men (only occasionally women), who wilfully destroy themselves amid the American convoys, outside the Iraqi police stations, in markets and around mosques and in shopping streets and on lonely roads beside remote checkpoints across the huge cities and vast deserts of Iraq. Never have the true figures for this astonishing and unprecedented campaign of self-liquidation been calculated.

But a month-long investigation by The Independent, culling four Arabic-language newspapers, official Iraqi statistics, two Beirut news agencies and Western reports, shows that an incredible 1,121 Muslim suicide bombers have blown themselves up in Iraq. This is a very conservative figure and – given the propensity of the authorities (and of journalists) to report only those suicide bombings that kill dozens of people – the true estimate may be double this number. On several days, six – even nine – suicide bombers have exploded themselves in Iraq in a display of almost Wal-Mart availability. If life in Iraq is cheap, death is cheaper.

This is perhaps the most frightening and ghoulish legacy of George Bush's invasion of Iraq five years ago. Suicide bombers in Iraq have killed at least 13,000 men, women and children – our most conservative estimate gives a total figure of 13,132 – and wounded a minimum of 16,112 people. If we include the dead and wounded in the mass stampede at the Baghdad Tigris river bridge in the summer of 2005 – caused by fear of suicide bombers – the figures rise to 14,132 and 16,612 respectively. Again, it must be emphasised that these statistics are minimums. For 529 of the suicide bombings in Iraq, no figures for wounded are available. Where wounded have been listed in news reports as "several", we have made no addition to the figures. And the number of critically injured who later died remains unknown. Set against a possible death toll of half a million Iraqis since the March 2003 invasion, the suicide bombers' victims may appear insignificant; but the killers' ability to terrorise civilians, militiamen and Western troops and mercenaries is incalculable.

Never before has the Arab world witnessed a phenomenon of suicide-death on this scale. During Israel's occupation of Lebanon after 1982, one Hizbollah suicide-bombing a month was considered remarkable. During the Palestinian intifadas of the 1980s and 1990s, four per month was regarded as unprecedented. But suicide bombers in Iraq have been attacking at the average rate of two every three days since the 2003 Anglo-American invasion.

And, although neither the Iraqi government nor their American mentors will admit this, scarcely 10 out of more than a thousand suicide killers have been identified. We know from their families that Palestinians, Saudis, Syrians and Algerians have been among the bombers. In a few cases, we have names. But in most attacks, the authorities in Iraq – if they can still be called "authorities" after five years of catastrophe – have no idea to whom the bloodied limbs and headless torsos of the bombers belong.

Even more profoundly disturbing is that the "cult" of the suicide bomber has seeped across national frontiers. Within a year of the Iraqi invasion, Afghan Taliban bombers were blowing themselves up alongside Western troops or bases in Helmand province and in the capital Kabul. The practice leached into Pakistan, striking down thousands of troops and civilians, killing even the principal opposition leader, Benazir Bhutto. The London Tube and bus bombings – despite the denials of Tony Blair – were obviously deeply influenced by events in Iraq.

Academics and politicians have long debated the motives of the bombers, the psychological make-up of the men and women who cold-bloodedly decide to undertake the role of suicide executioners; for they are executioners, killers who see their victims – be they soldiers or civilians – before they flick the switch that destroys them. The Israelis long ago decided that there was no "perfect" profile for a suicide bomber, and my own experience in Lebanon bears this out. The suicider might have spent years fighting the Israelis in the south of the country. Often, they would have been imprisoned or tortured by Israel or its proxy Lebanese militia. Sometimes, brothers or other family members would have been killed. On other occasions, the example of their own relatives would have drawn them into the vortex of suicide-by-example.

Khaled is – or was, for I no longer know if he is alive, since I met him a few weeks ago– influenced by his brother Hassan, whose journey to Iraq was organised by an unknown group, presumably Palestinian, and whose weapons training beside the Tigris river was videotaped by his comrades. Hassan's mother has shown me this tape – which ends with Hassan cheerfully waving goodbye from the driver's window of a battered car, presumably the vehicle he was about to ram into the American convoy at Tal Afar.

None of this addresses the issue of religious belief. While there is evidence aplenty that the Japanese suicide pilots of the Second World War were sometimes coerced and intimidated into their final flights against US warships in the Pacific, many also believed that they were dying for their emperor. For them, the fall of cherry blossom and the divine wind – the "kamikaze" – blessed their souls as they aimed their bombers at American aircraft carriers. But even an industrialised dictatorship like Japan – facing the imminent collapse of its entire society at the hands of a superpower – could only mobilise 4,615 "kamikazes". The Iraq suicide bombers may already have reached half that number.

But the Japanese authorities encouraged their pilots to think of themselves as a collective suicide unit whose insignia of imminent death – white Rising Sun headbands and white scarves – prefigured the yellow headbands imprinted with Koranic script that Hizbollah guerrillas wore when they set out to attack Israeli soldiers in the occupied zone of southern Lebanon. In Iraq, however, those who direct the growing army of suiciders do not lack inventiveness. Their bombers have arrived at the scene of their self-destruction dressed as car mechanics, soldiers, police officers, middle-aged housewives, children's sweet-sellers, worshippers and – on one occasion – a "harmless" shepherd. They have carried their bombs in Oldsmobiles, fuel trucks, garbage trucks, flat-bed trucks, on donkeys and bicycles, motor-bikes and mopeds and carts, minibuses, date-vendors' vans, mobile recruitment centres and lorries packed with chlorine. Incredibly, there appears to be no individual central "brain" behind the bombings – although "groupuscules" of bombers obviously exist. Inspiration, imitation and the globalised influence of the internet appear sufficient to empower the bombers of Iraq.

On an individual level, it is possible to see the friction and psychological trauma of families. Khaled's mother, for instance, constantly expressed her pride in her dead son Hassan and, in front of me, she looked with almost equal love at his still-living brother. But when my companion urged Khaled to remain alive for his mother's sake – reminding him that the Prophet himself spoke of the primary obligation of a Muslim man to protect his mother – the woman was close to tears. She was torn apart by her love as a mother and her religious-political duty as the woman who had brought another would-be martyr into the world. When my friend again urged Khaled to remain alive, to stay in Sidon and marry – eerily, the muezzin's call to prayer had begun during our conversation – he shook his head.

Not even a disparaging remark about those who would send him on his death mission – that they were prepared to live in this world while sending others like Khaled to their fate – could discourage him. "I am not going to become a 'shahed' [martyr] for people," he replied. "I am doing it for God."

It was the same old argument. We could produce a hundred good ways – peaceful ways – for him to resolve the injustices of this world; but the moment Khaled invoked the name of God, our suggestions became irrelevant. Rationality – humanism, if you like – simply withered away. If a Western president could invoke a war of "good against evil", his antagonists could do the same.

But is there a rational pattern to the suicide bombings in Iraq? The first incidents of their kind took place as American troops were actually advancing towards Baghdad. Near the Shia town of Nasiriyah, an off-duty Iraqi policeman, Sergeant Ali Jaffar Moussa Hamadi al-Nomani, drove a car bomb into an American Marine roadblock. Married, with five children, he had been a soldier in Iraq's 1980-88 war with Iran and had volunteered to fight the Americans after Saddam's occupation of Kuwait. Shortly afterwards, two Shia Muslim women did the same.

In its dying days, even Saddam Hussein's own government was shocked. "The US administration is going to turn the whole world into people prepared to die for their nations," Saddam's vice-president, Taha Yassin Ramadan, warned. "All they can do now is turn themselves into bombs. If the B-52 bombs can now kill 500 or more in our war, then I'm sure that some operations by our freedom fighters will be able to kill 5,000." Ramadan even referred to "the martyr's moment of sublimity" – an al-Qa'ida-like phrase that ill befitted a secular Baathist – and it was clear that the vice-president was almost as surprised as the Americans. But only two days after the US occupation of Baghdad, a woman killed herself while trying to explode a grenade among a group of American troops outside the capital.

Throughout the five years of war, suicide bombers have focused on Iraq's own American-trained security forces rather than US troops. At least 365 attacks have been staged against Iraqi police or paramilitary forces. Their targets included at least 147 police stations (1,577 deaths), 43 army and police recruitment centres (939 deaths), 91 checkpoints (with a minimum of 564 fatalities), 92 security patrols (465 deaths) and numerous other police targets (escorts, convoys accompanying government ministers, etc). One of the recruitment centres – in the centre of Baghdad – was assaulted by suicide bombers on eight separate occasions.

By contrast, suicide bombers have attacked only 24 US bases at a cost of 100 American dead and 15 Iraqis, and 43 American patrols and checkpoints, during which 116 US personnel were killed along with at least 56 civilians, 15 of whom appear to have been shot by American soldiers in response to the attacks, and another 26 of whom were children standing next to a US patrol. Most of the Americans were killed west or north of Baghdad. Suicide attacks on the police concentrated on Baghdad and Mosul and the Sunni towns to the immediate north and south of Baghdad.

The trajectory of the suicide bombers shows a clear preference for military targets throughout the insurgency, with attacks on Americans gradually decreasing from 2006 and individual attacks on Iraqi police patrols and police recruits increasing over the past two years, especially in the 100 miles north of Baghdad. Just as the Islamist murderers of Algeria – and their military opponents – favoured the fasting month of Ramadan for their bloodiest assaults in the 1990s, so the suicide bombers of Iraq mobilise on the eve of religious festivals. There was a pronounced drop in suicide assaults during the period of sectarian liquidations after 2005, either because the bombers feared interception by the throat-cutters of tribal gangs working their way across Baghdad, or because – a grim possibility – they were themselves being used in the sectarian murder campaign.

The most politically powerful attacks occurred inside military bases – including the Green Zone in Baghdad (two in one day in October 2004) – and against the UN headquarters (in which the UN envoy Sergio de Mello was killed) and the International Red Cross offices in Baghdad in 2003. By December 2003, British officials were warning that there were more "spectacular" suicide bombings to come, and the first suicide assault on a mosque took place in January of the following year when a bomber on a bicycle blew himself up in a Shia mosque in Baquba, killing four worshippers and wounding another 39.

Scarcely a year later, another suicider attacked a second Shia mosque, killing 14 worshippers and wounding 40. In February 2004, a man blew himself up on a bus outside the Shia mosque at Khadamiyah in Baghdad, killing 17 more Shia Muslims. Only a few days earlier, a man wearing an explosives belt killed four at yet another Shia mosque in the Doura district of Baghdad. The suicide campaign against Shia places of worship continued with an attack on a Mosul mosque in March 2005, killing at least 50, two more attacks in April that killed 26, and another in May in Baghdad.

While Shia mosques were being targeted in a deliberate campaign of provocation by al-Qa'ida-type suiciders, markets and hospitals frequented by Shia Muslims were also attacked. Almost all the 600 Iraqis killed by suicide bombs in May 2005 were Shias. After the partial demolition of the Shia mosque at Samarra on 22 February 2006, the "war of the mosques" began in earnest for the suicide bombers of Iraq. A Sunni mosque was blown up, with nine dead and "dozens" of wounded, and two Shia mosques were the target of suicide bombers in the same week. In early July 2006, seven suicide killers blew themselves up in Sunni and Shia mosques, leaving a total of 51 civilians dead. During the same period, a suicide bomber launched the first attack of its kind on Shia pilgrims arriving from Iran.

Bombers were to attack the funerals of those Shia they had killed, and even wedding parties. Schools, university campuses and shopping precincts were also now included on the target lists, most of the victims yet again being Shia. Over the past year, however, an increasing number of tribal leaders loyal to the Americans – including Sattar Abu Risha, who publicly met President Bush on 13 September 2007, and former insurgents who have now joined the American-paid anti-al-Qa'ida militias – have been blown apart by Sunni bombers.

Only about 10 of the suicide bombers have been identified. One of them, who attacked an Iraqi police unit in June 2005, turned out to be a former police commando called Abu Mohamed al-Dulaimi, but the Americans and the Iraqi authorities appear to have little intelligence on the provenance of these killers. On at least 27 occasions, Iraqi officials have claimed to know the identity of the killers – saying that they had recovered passports and identity papers that proved their "foreign" origin – but they have never produced these documents for public inspection. There is even doubt that the two suicide bombers who blew themselves up in a bird market earlier this year were in fact mentally retarded young women, as the government was to allege.

Indeed, nothing could better illustrate the lack of knowledge of the authorities than the two contradictory statements made by the Americans and their Iraqi protégés in March of last year. Just as David Satterfield, US Secretary of State Condoleezza Rice's adviser on Iraq, was claiming that "90 per cent" of suicide bombers were crossing the border from Syria, Iraq's Prime Minister, Nuri al-Maliki, was announcing that "most" of the suiciders came from Saudi Arabia – which shares a long, common border with Iraq. Saudis would hardly waste their time travelling to Damascus to cross a border that their own country shared with Iraq. Many in Baghdad, including some government ministers, believe that the nationality of the bombers is much closer to home – that they are, in fact, Iraqis.

It will be many years before we have a clearer idea of the number of bombers who have killed themselves in the Iraq war – and of their origin. Long before The Independent's total figure reached 500, al-Qa'ida's Abu Musab al-Zarqawi was boasting of "800 martyrs" among his supporters. And since al-Zarqawi's death brought not the slightest reduction in bombings, we must assume that there are many other "manipulators" in charge of Iraq's suicide squads.

Nor can we assume the motives for every mass murder. Who now remembers that the greatest individual number of victims of any suicide bombing died in two remote villages of the Kahtaniya region of Iraq, all Yazidis – 516 of them slaughtered, another 525 wounded. A Yazidi girl, it seems, had fallen in love with a Sunni man and had been punished by her own people for this "honour crime": she had been stoned to death. The killers presumably came from the Sunni community.

One of George Bush's most insidious legacies in Iraq thus remains its most mysterious; the marriage of nationalism and spiritual ferocity, the birth of an unprecedentedly huge army of Muslims inspired by the idea of death.