As people bustled through the Saturday market in the Nigerian city of Maiduguri, a device borne by a ten year-old girl exploded near the entrance.
A witness said the girl probably had no idea that a bomb had been strapped to her body.
The explosion just before lunch killed 20, including the girl, and injured 18, according to the police.
Boko Haram did not immediately claim the attack, but the Islamic insurgents have increasingly used young girls as human bombs as they carve an African “Caliphate” from the plains of northern Nigeria.
Within this domain, the black flag of jihad flies over scores of towns and villages scattered across the neighbouring states of Borno and Yobe.
The latest conquest was the fishing town of Baga on the shores of Lake Chad, which fell to the Islamists last Wednesday.
“For five kilometres (three miles), I kept stepping on dead bodies until I reached Malam Karanti village, which was also deserted and burnt,” one surviving fisherman, Yanaye Grema, said.
Boko Haram’s fighters have now achieved mastery over 11 local government areas with a total population exceeding 1.7 million people, according to the official 2006 census.
Once, the movement’s fighters would launch hit-and-run attacks on defenceless villages. Now, Boko Haram’s realm stretches from the Mandara Mountains on the eastern border with Cameroon to Lake Chad in the north and the Yedseram river in the west.
The Nigerian army, crippled by corruption and incompetence, has shown itself unable to resist the jihadist advance.
Last September, Abubakar Shekau, the self-styled “Emir” of Boko Haram, proclaimed his ambition to conquer a “Caliphate” and follow the example of the Islamic State of Iraq and the Levant (Isil).
Diplomats believe this was a logical escalation of Boko Haram’s campaign.
“There is a copy-cat element at work here,” said Andrew Pocock, the British High Commissioner to Nigeria. “If Isil can declare a Caliphate, then so can we. Boko Haram want to be seen by their peers as grown-up jihadis. They want to show 'we can control territory, we can control a Caliphate’.”
There is also a clear practical rationale for Boko Haram to capture territory. “Success - and they have had success - creates a different kind of requirement,” added Mr Pocock. “You need a place where you can base yourself and keep equipment and supplies and, indeed, captives. It means that you’ve got to hold territory.”
Shekau has established Boko Haram’s unofficial headquarters in the town of Gwoza in Borno state. This stronghold has been chosen with great care.
Gwoza is shielded from attack by the volcanic peaks of the Mandara Mountains spanning the nearby frontier with Cameroon. Most important of all, the surrounding area is the homeland of Shekau’s own ethnic group, the Kanuri.
From this base among his brethren, Shekau sends his fighters to strike across a vast area. The border with Cameroon means nothing to Shekau, since it slices directly through the area inhabited by the Kanuri. His men have frequently attacked villages in the neighbouring country, killing 68 of Cameroon’s soldiers in the last month alone.
Sometimes, Shekau’s goal is to grab more territory - as with the assault on Baga last week. Just as often, he dispatches his fighters on what can only be described as slave raids.
Boko Haram profits greatly from the trade in human beings. Last April, Shekau committed his most infamous act by abducting over 200 schoolgirls from the town of Chibok, about 50 miles south-west of Gwoza, triggering a global campaign to “bring back our girls”. By his own admission, the girls were then sold into slavery.
Britain and France stamped out the slave trade in this part of Africa a century ago, but Boko Haram has succeeded in partially reversing this achievement. Today the ancient caravan routes running north across the Sahara are active once more, except that trucks have replaced camels as the means of conveying human cargo.
Boko Haram has expanded to a point where it defies simple categories. Its name is normally translated as “Western education is banned”, yet “boko” means “book” in the Hausa language, so “books are banned”would be more accurate.
In part, Boko Haram is a branch of al-Qaeda’s brand of jihadism. As well as seizing towns, Shekau’s men carry out suicide bombings in Nigerian cities, including Abuja. Like the Taliban in Afghanistan and Isil in Iraq, they have become expert users of improvised explosive devices. In particular, its men have mastered the technique of creating charges that are carefully shaped to destroy armoured vehicles.
In short, Boko Haram have learnt the classic tactics of al-Qaeda. Its operatives have picked up these skills from the jihadist tentacles stretching across the Sahel region of Africa. “Unquestionably, Boko Haram have benefited from the broader Sahelian jihadi network,” said Mr Pocock.
Yet at the same time, Boko Haram is a Kanuri tribal insurgency. In addition, the movement works as a criminal gang, profiting from theft, extortion and slave raiding. Shekau amounts to a global jihadist, crime boss and tribal rebel leader - all at the same time.
If there are limits to his ambitions, they have not been imposed by the Nigerian army. The 7th division was specially created to fight Boko Haram and deployed to Borno. In practise, it does little but try to mount a static defence of Maiduguri, the state capital. In common with the rest of the army, it lacks the mobility and the manpower to challenge Boko Haram’s control of the surrounding area.
The army may also lack the resolve. Last year, the federal government allocated 20 per cent of its budget to the armed forces - over £4 billion.
Yet precious little trickled down to the soldiers in the frontline, who remain poorly armed and equipped. Instead, a large proportion of the military budget simply disappeared into the pockets of senior officers.
Despite being the headquarters of the 7th division, Maiduguri lives under the constant threat of attack.
Oliver Dashe Doeme, the Roman Catholic Bishop in the city, said that 70 of the 150 churches in his diocese had been destroyed by Boko Haram. “We have many parishes which have been sacked and overrun,” he said. “Our major concern is not our buildings - it’s not the churches themselves - but our people who have been driven away from their homes. Some are living in mountains and forests, some are in Cameroon and some have gone elsewhere in Nigeria.”
About 10,000 Catholic refugees have gathered in Maiduguri after fleeing Boko Haram’s new domain, added Bishop Doeme.
President Goodluck Jonathan, who faces re-election next month, has declared an emergency in the three states most threatened by the Islamists. But Bishop Doeme has no confidence in the army’s ability to recapture the territory lost to Boko Haram.
“Nigeria is a very corrupt nation,” he said. “Our main problem is not that Boko Haram cannot be contained, but that you have a deep-seated corruption in high and low places. Many of our top military officers are gaining from what is happening here because it means that a lot of money is coming in their direction.”
Instead of being imposed by the army, the borders of Boko Haram’s new domain may be defined by the ethnic patchwork of northern Nigeria.
Shekau is confident of holding the territory inhabited by his fellow Kanuris, but his grip is looser wherever other groups hold sway.
He recently withdrew from a string of towns in Adamawa state, perhaps because they were inhabited by non-Kanuris.
Boko Haram possesses armoured personnel carriers, anti-aircraft guns and rocket launchers - in other words, the heavy weapons of a conventional army. Despite all this firepower, the invisible borders of ethnicity may still be a brake on its expansion.
For Bishop Doeme, however, this comes as little consolation. “We have cases here of soldiers deliberately abandoning their armaments,” he said.
“There are cases of corrupt senior officers. The president should sack them so there is an example to the others - but that has never happened.”