The group behind these daring kidnappings calls itself Boko Haram, a name that loosely translates as "Western education is a sin" in the local Hausa language of north-central Africa.
The group is part of a jihadist Islamist organization that has been linked to al-Qaeda and is responsible for a series of increasingly brazen attacks in Nigeria, currently the richest country in Africa by GDP.
In September 2013, Boko Haram waged an attack on a college in Gujba, Yobe state, in the middle of the night, killing 44 male students and teachers while they were sleeping.
Three weeks ago, Boko Haram claimed responsibility for an early-morning bomb blast at a bus terminal in the Nigerian capital, Abuja, that killed 71 people and wounded 124.
And then there are the kidnappings.
On April 14, about 300 girls from different institutions had gathered at a girls' school in Chibok, Borno state, in northeastern Nigeria, near the border with Cameroon and Chad. Most were between 16 and 18, and they were there to take their final exams.
Around midnight, a group of men appeared dressed in military garb and claiming to be soldiers. They told the girls they were there to shield them from an imminent attack by Boko Haram, and whisked them away to an undisclosed location. A few dozen of the girls are said to have escaped.
Earlier this week, Boko Haram confirmed they themselves were behind the attack, and said the girls would be "sold" in the marketplace; some said the would be sold as wives to the jihadist soldiers in the group.
On Tuesday, Boko Haram announced it had kidnapped eight more girls from a village near their base in northeastern Nigeria.
When was Boko Haram formed?
Boko Haram was founded in 2002 by Mohammad Yusuf, an Islamic cleric in Maiduguri, the capital of the northeastern state of Borno.
Initially a social movement, Boko Haram claimed the widespread poverty and dire schooling in the country's largely Muslim north was the result of federal neglect, says Daniel Douek, a professor at Concordia University with a specialty in African politics.
Boko Haram has arisen "in a historically marginalized region of Nigeria," says Douek. "The Borno state is the poorest region in Nigeria, and receives the least government money."
Douek says many northerners also harbor resentments about the legacy of British rule and maintain a longstanding resistance to the Nigerian government.
In fact, many in Borno see the federal government, including the current presidency of Goodluck Jonathan, "as a kind of continuation of the colonial power that preceded it," Douek says.
When did the organization become violent?
Nigerian authorities began to clamp down on the group in the mid-2000s, in response to reports that Boko Haram was stockpiling weapons, says Christopher Anzalone of the Institute of Islamic Studies at McGill University.
In 2009, authorities took Yusuf into custody, where he was killed by police. The video of his execution went viral, intensifying anger towards the Nigerian authorities and radicalizing many in the north.
Under the watch of Yusuf's successor, AbubakarShekau, the group has stepped up its campaign. There have been numerous strikes on schools, as well as an attack on a UN building in Abuja in 2011 that killed 21 people.
The Nigeria Security Tracker, a statistical tool maintained by the Council on Foreign Relations, shows that killings attributed to Boko Haram have risen dramatically in the last couple of years, with over 2,100 alone having died the week of March 23, 2014.
What are their goals?
The group's aim is to establish an Islamic state governed by sharia law in northern Nigeria.
Whether it's bombing markets or attacking schools, Douek says the group's assaults have a dual purpose: intimidating those who wish to pursue a Western-style lifestyle or education, and demonstrating that the Nigerian state is unequipped to protect them.
"What they are doing is attacking the secular Nigerian state, which they say is utterly corrupt," John Campbell, a former American ambassador to Nigeria and author of Nigeria: Dancing on the Brink, told CBC Radio's The Current recently.
"They completely reject secularism. They say government of the people, by the people and for the people will be destroyed and replaced by government of Allah, by Allah and for Allah."
While Boko Haram harbors an antipathy towards the Christian-majority south, most of its victims have been fellow Muslims in the north.
The ostensible reason for this, Anzalone says, is that Boko Haram views the traditional Muslim leadership in the northern states as corrupt or as competition — or both.
What is their leadership structure?
Experts say that after the death of Mohammad Yusuf, the group's leadership splintered, although little is known about any of Boko Haram's other principals.
Its most visible member now is Abubakar Shekau, who has appeared in a series of videos released by the group, including the recent one in which the group took responsibility for the abduction of the schoolgirls.
Based on the videos alone, Shekau seems to possess "military know-how" and "a strong religious education," says Douek.
While the group has claimed responsibility for a variety of attacks, it might be possible to overestimate its reach, say some analysts. In a 2012 op-ed piece in the New York Times, Nigeria scholar Jean Herskovits suggested that Nigerian criminal gangs "have adopted the name Boko Haram to claim responsibility for attacks when it suits them."
How has the Nigerian government dealt with the situation?
"One of the difficulties of dealing with Boko Haram is, because it's not a conventional political movement, conventional tactics don't work," says Campbell.
"There have been no negotiations. There have been efforts by the government to negotiate over the years. But whenever someone who claims to be part of Boko Haram has involved himself in such talks, he has normally been killed the following day."
According to Douek, the government has made the situation worse with its poor intelligence and "heavy-handed" attacks that are meant to snuff out Boko Haram but more often than not end up killing innocent civilians.
"This kind of punishment has embittered sectors of the population in the Nigerian North in ways that have played into Boko Haram's hands, and according to some have boosted their recruitment," Douek says.