The failure of #BringBackOurGirls: Boko Haram kidnapped 276 girls two years ago. What happened to them?

Two years ago, just before midnight on a sweltering night in a town in northeastern Nigeria, men carrying AK-47s stormed into the Chibok Government Secondary School.

What happened next would bring global attention to the Islamist group Boko Haram, which had been haunting Nigeria for years. It would unite activists around the world, including first lady Michelle Obama, around the hashtag #BringBackOurGirls. It would prompt the United States to dispatch surveillance drones and military trainers to West Africa.

The militants kidnapped 276 schoolgirls. Several dozen of them were able to escape.

Where are they?But two years later, even as the Nigerian, Cameroonian and Chadian militaries have pushed Boko Haram out of many of its former strongholds, 219 of the girls remain missing.

"I assure you that I go to bed and wake up every day with the Chibok girls on my mind," Nigeria's president, Muhammadu Buhari, said earlier this year.

But his efforts have not resulted in the return of any of the girls. Thousands of other Boko Haram victims have been released, recounting stories of forced marriage and sexual slavery. But not the Chibok girls. Most Nigerian and Western officials say the girls have been taken to a remote part of the Sambisa forest, a former game reserve in northeastern Nigeria, where they are being closely guarded by the militants.

New footage obtained by CNN, thought to have been shot last December, shows around 15 young women are seen bowing their heads and looking scared as they hesitantly answer his questions. It is evidence that at least some of the girls may still be alive and are being held captive by Boko Haram militants.

The footage was shared with the girls' parents, and in the CNN video they can be seen bursting into tears as they recognise their daughters and finally have some hope to cling to.

"My Saratu!" cries one mother, reaching out to touch the image of her 17-year-old on the laptop screen.

In fact, there have been repeated signs the girls are still alive and in captivity somewhere in Nigeria.

The initial failure of Nigerian officials and the military to rescue the girls brought international condemnation and contributed to President Goodluck Jonathan's loss in elections last year.

Michelle Obama was one of hundreds of celebrities and high-profile figures to lend their support to the #BringBackOurGirls campaign.

He at first denied there had been a mass abduction, but was soon forced to accept help from other countries. The United States, Britain and France were among those who sent advisers, including hostage negotiators.

US and British drones located at least one group of about 80 of the girls, but Nigeria's government and military took no action.

Andrew Pocock, British high commissioner to Nigeria until last year, told The Sunday Times magazine last month that the military feared the girls might be hurt if they attempted a ground or air rescue. "You might have rescued a few, but many would have been killed," he said. "You were damned if you do and damned if you don't."

Nigeria's military has cited the same fears. Yet they have attacked towns and villages where Boko Haram has held thousands of civilians captive, reporting last week that soldiers had rescued 11,595 hostages since February 26.

Here's what we know about Boko Haram's reign that might help us understand what the missing girls have endured, and what would await them if they are rescued.

1. While militants have publicly accused Nigeria's secular government of being an affront to Islam, their victims say the group's members appeared much more interested in expanding their campaign of sexual violence than governing their self-proclaimed caliphate. Many of Boko Haram's victims became pregnant while in captivity. Most experts expect that the majority of the Chibok girls have already delivered the babies of their captors. And if they are released, like the other victims, they will likely be regarded with deep suspicion.

2. Boko Haram has leveled a number of the cities and villages that it once occupied, leaving more than 2 million people without homes. In Chibok, homes were burned to the ground. The secondary school was destroyed. The Nigerian government says it has begun rebuilding the part of the country ravaged by war: a multibillion-dollar project. But so far there is little evidence of progress.

3. Some girls abducted by Boko Haram have recently carried out suicide bombings. The number of children involved in such blasts grew tenfold, from four in 2014 to 44 in 2015, according to a report released by the U.N. children's agency on Tuesday. Many Nigerians wonder: Were the Chibok girls brainwashed by their captors? Will they be forced into becoming attackers themselves?

4. The Nigerian military's campaign to dislodge Boko Haram might not be enough to locate the girls. The army has proven itself capable of conducting offensives in cities and towns, but the Sambisa forest is another tactical challenge entirely - dense, remote and vast. The leader of a different group, the Lord's Resistance Army, Joseph Kony, has been lurking for years near the borders of Sudan, South Sudan and the Central African Republic. The forces trying to find him include 100 U.S. troops. But the effort has yielded nothing. Some say that's a cautionary tale for the quest to find the Chibok girls and Boko Haram's top leadership. That top leadership, by the way, includes a number of teenage boys, according to the Nigerian military.