Hold the bomb under your armpit to keep it steady, the women and girls were taught. Sever your enemy’s head from behind, to minimize struggling.

“If you cut from the back of the neck, they die faster,” said Rahila Amos, a Nigerian grandmother describing the meticulous instruction she received from Boko Haram to become a suicide bomber.

Of all the many horrors of Boko Haram’s rampage across West Africa — the attacks on mosques, churches and schools; the mass killings of civilians; the entire villages left in ashes after militants tear through — one of the most baffling has been its ability to turn captured women and girls into killers, reports The New York Times..

Boko Haram, one of the world’s deadliest extremist groups, has used at least 105 women and girls in suicide attacks since June 2014, when a woman set off a bomb at an army barracks in Nigeria, according to The Long War Journal, which tracks terrorist activity.

Since then, women and girls, often with bombs hidden in baskets or under their clothes, have killed hundreds of people in attacks on fish and vegetable markets, schools, a river dock and even camps for people who have fled their homes to escape the violence.

“This isn’t something you can defeat or eradicate outright,” said Issa Tchiroma Bakary, the minister of communications in Cameroon, where 22 female suicide bombers were identified since the start of this year. “You don’t know who is who. When you see a young girl moving toward you, you don’t know if she’s hiding a bomb.”

Soldiers cannot open fire on every woman or girl who looks suspicious, he added. “They know where we have the Achilles’ heel,” Mr. Bakary said of Boko Haram.

Boko Haram’s abuse of women first shocked the world two years ago, when it stormed a school in Nigeria and fled with about 300 girls, many of whom were never found. Hundreds of other women and girls have been abducted, imprisoned, raped and sometimes intentionally impregnated, perhaps with the goal of creating a new generation of fighters.

Ms. Amos, 47, said the fighters had come to her village in the morning, firing weapons as they spilled out of cars and rounded up women and children.

Not long after, Ms. Amos, a Christian, said she was forced to enroll in Boko Haram’s classes on its version of Islam, a first step on her way toward being taught the art of suicide bombing.

After months of training, Ms. Amos said she was finally able to escape her captors one day when they had assembled for evening preaching. She stayed behind, gathering two of her young children and a grandchild so they could make a run for the Cameroonian border.

“I don’t want to take a bomb,” she said inside this refugee camp in Cameroon that stretches across a vast landscape dotted by tents and mud huts.

The authorities in Cameroon and Nigeria said that many of the experiences detailed by Ms. Amos matched the accounts of other women and girls who have escaped Boko Haram, or who have been arrested before they could detonate bombs. Ms. Amos’s assertions are also strikingly similar to details recounted by other freed women and girls, including descriptions of the funeral rites performed before female bombers were sent on missions.

The accounts offer insight into how Boko Haram, despite being under military pressure from a multinational campaign to wipe it out, has been able to strike fear across an expansive battlefield that now includes Nigeria, Chad, Cameroon and Niger.

No longer able to control the territory as tightly it once did, Boko Haram is sending out women and young girls as newly minted terrorists who can inflict a devastating toll.

Col. Didier Badjeck, a Cameroon defense spokesman, said that after soldiers had chased Boko Haram out of villages in recent weeks, they found homes that had been used as prisons for the women and girls. He said female hostages had reported being trained during their captivity — both in the Quran and in violence.

“They are training them to maximize the number of victims,” Colonel Badjeck said. “We are sure about it.”

Boko Haram often sends male fighters to set upon mosques. But last month, a woman dressed as a man set off her explosives during morning prayers in a village in northeastern Nigeria. Another woman was waiting outside the mosque, and as people fled the first blast, she detonated her own explosives as well. In all, at least 24 people were killed.

Bombings by women have become so widespread that even humanitarian groups are rethinking how they distribute food, water and other help to them. What if one of the women is hiding a bomb?

In Cameroon, many of the recent bombings have been carried out by girls in their early teens, leaving officials and analysts to wonder whether the girls were aware they were carrying bombs. Yet some of the bombers in recent attacks in Nigeria have been found to wear their hair pulled back from the face — a hairstyle reserved for burial rites, a sign they were ready to die.

But cracks are starting to show in the Boko Haram suicide-training system. In February, a girl sent to bomb a village in the Far North Region of Cameroon dropped her explosives and ran to the authorities instead. Her information led to a major raid on Boko Haram fighters.

In northeastern Nigeria in February, three girls with bombs were sent into a camp for Nigerians fleeing Boko Haram. Two of the girls detonated their bombs, killing nearly 60 people there. But the third girl spotted her parents among the desperate people inside the camp. Overwhelmed, officials said, she threw her explosives in the bush.

Boko Haram, which pledged allegiance to the Islamic State last year, has abducted as many as 2,000 women and children, both girls and boys, since 2012, according a recent report from humanitarian groups. Young boys have been used as bombers, too.

In many ways, female bombers are ideal weapons. At security points run by men, they are often searched less thoroughly, if at all. Tucked under the bunched fabric of dresses or religious gowns, explosives are easy to conceal.

Female suicide bombers have been a trademark of extremists for decades. In the Chechnya conflict, they were nicknamed black widows. In Sri Lanka, they fought with the Tamil Tigers. In her book, “Bombshell: Women and Terrorism,” Mia Bloom estimates that between 1985 and 2008, women committed a quarter of all suicide bombings.

One soldier who has engaged with Boko Haram said he believed that fighters must drug the girls’ food. Others who track the group question whether the bombs are remotely detonated.

According to Ms. Amos, Boko Haram’s use of women as weapons is a carefully thought-out strategy, one some of the women accept. Ms. Amos said that of the 30 or so female captives enrolled in training with her, seven girls were enthusiastic about carrying out suicide missions.

“It was a direct path to heaven,” she said the group was told.

Ms. Amos, now among the 58,000 residents of the Minawao Refugee Camp, described a system of grooming potential bombers that involved food deprivation and promises of eternal life, tactics that cults have used for decades.

She said that when Boko Haram stormed her hometown in 2014, her two brothers were shot dead. Her husband managed to flee with five of their children, but Ms. Amos did not make it out, and neither did two of their other young children and a grandchild. Boko Haram rounded them up with other women and children, putting them in a long ditch to contain them.

They stayed there for days, eating one meal a day of a corn paste made from powder. Finally a fighter arrived and asked a fateful question: Do you want to follow Christ, or do you want to be a Muslim?

The women all agreed to follow Islam, fearing they would be killed otherwise. Their training began. Ms. Amos described a six-tiered daily education track for the women that she called Primary One, Primary Two and so on. The first two levels were Quranic training. Primary Three was training in suicide bombing and beheading. “How to kill a person and how to bomb a house,” she said.

“They told us if we came upon a group of 10 to 20 people to press this,” she said, speaking of a detonator.

The instruction given in the upper levels of the training — Primary Four, Five and Six — was a closely guarded secret among the fighters. Ms. Amos said she never learned what took place there.

Ms. Amos was lucky. Boko Haram fighters decided not to “marry” her, a euphemism for the rapes the group commits, because she already had a husband and children. She counted 14 women and four girls in her training classes who were not as fortunate.

Throughout her months in captivity, Ms. Amos was fed one meal a day and lost weight, a fact confirmed by her nephew living in the Minawao camp, who stared at her scrawny frame and said, “She used to be a big woman.”

Boko Haram incorporated the lack of food into the training, Ms. Amos said. Several months ago, she said, fighters rounded up the women and took them to an old factory to view a set of plump, well-fed girls who had plenty of food and water. Follow our ways, the fighters said, and you can have enough to eat, like these girls.

The girls, some of them crying, told Ms. Amos they were from Chibok, the Nigerian village where Boko Haram had captured the schoolgirls. American State Department and military officials said they would investigate the statements from Ms. Amos about the girls.

“They were very fat,” Ms. Amos said, compared with herself and the other women who were being held, “and they had lots of water.”

Photo: Rahila Amos, a Nigerian grandmother, said she was abducted by Boko Haram and forced to take classes on how to carry out a suicide bombing. Credit Tyler Hicks/The New York Times