In a year packed with terrorist attacks, the world's deadliest extremist group has carried out massacres the size of the San Bernardino, Calif., killings once or twice a week. And over time, it has undertaken dozens of attacks that dwarf November's deadly rampage in Paris, sometimes shooting down several hundred civilians at a time.

Boko Haram, the Nigerian Islamist group, has been more deadly than Islamic State. And every time Nigeria's army seems to have made substantial progress toward wiping it out, the group has quietly rebuilt. Its members cut the throats of schoolboys, casting them aside to bleed to death. And they behead victims, like Islamic State, and record the atrocities on video.

Although Boko Haram has at times threatened the West, it has largely focused on poor Nigerian villagers, far from the media spotlight.

Five years ago, in a clandestine interview in Kano, a leader of Boko Haram described acts of terrorism against the U.S. as "divine worship."

"They are fighting Islam, and we will also fight them, if we get the chance," he said.

Boko Haram, modeled on Afghanistan's Taliban, was at its lowest ebb in 2010, with Nigerian authorities confident they had brought the organization to its knees after having killed 700 of its fighters in a battle the previous year. But Boko Haram went underground, regrouped, and has since launched thousands of attacks. Last year, it was the world's deadliest terrorist group, according to the Global Terrorism Index released recently by the Institute for Economics and Peace, a research group.

"It's proven to be one of the most resilient organizations. It's evolved quickly. It's shifted alliances. It's been pronounced dead numerous times," said J. Peter Pham, director of the Africa Center at the Atlantic Council, another research group.

"At one point it had no outside support. Then they got support from al-Qaeda. It dropped al-Qaeda and went over to the new winning team, ISIS," he said.

Boko Haram was blamed for recent twin suicide bombings in Kano, one carried out by an 11-year-old girl, and a market bombing in Yola that killed 34. The extremist group was responsible for 6,644 deaths in 2014, a 300 percent increase from the previous year, according to the Global Terrorism Index. In comparison, Islamic State killed 6,073 people in 2014.

The overall number of terrorism deaths increased from 18,111 in 2013 to 32,685 in 2014, the report said; the most terrifying place to live was in the northeast Nigeria region that is Boko Haram's home turf.

Victims of the group, and of others like Somalia's al-Shabaab, describe attackers displaying a cold, emotionless aura in some of the continent's worst terrorist attacks, including two in Kenya: the Westgate shopping mall massacre in 2013, in which 67 people were killed; and another at Garissa University College in April, in which 148 were killed, most of them students. Both demonstrated how much damage a few heavily armed suicidal men can cause in a short time.

As in Paris and San Bernardino, typical Boko Haram attacks target people going about their business.

Dozens of terrorist fighters swarm into a village or town on motorcycles or in pickup trucks and open fire on a market or square. In many attacks, hundreds of people have been killed, some of them burned alive, according to survivors. Men and boys as young as 10 would be dragged from their houses and shot, or slaughtered with knives.

Hauwa Umar saw men's beheaded bodies strewn about the town of Gwoza after Boko Haram attacked in August last year.

"There were uncountable bodies without heads," she said in a March interview. "Boko Haram kept saying, 'Stop crying! Stop crying!' I couldn't stop crying, and they'd shoot their guns in the air to shock you. But I kept on crying."

Women and children were abducted as sex slaves. (Hundreds have been released in recent months, but not 219 girls still missing from among 276 abducted from Chibok last year.)

Shehu Sani, a Nigerian senator and human-rights activist, has been involved in repeated efforts to negotiate a cease-fire with Boko Haram under President Goodluck Jonathan and his successor, Muhammadu Buhari. But the talks, also designed to secure the freedom of the kidnapped Chibok girls, have so far failed. Buhari set a deadline to crush the extremist group by December, but his office recently acknowledged that the effort would take longer.

Boko Haram has become more violent and more difficult to negotiate with since rebranding itself the Islamic State's West Africa Province this year, Sani said.

"They have stepped up attacks on soft targets, killing innocent people," he said.

The army's success in driving Boko Haram from its forest hideouts in recent months merely resulted in the group's going underground, moving to cities and launching attacks on civilians, Sani said.

"To date, they have not shown themselves to be a direct threat to Western countries," Pham said. "But I wouldn't rule it out in the future. It has evolved very quickly. That they haven't attacked foreign targets doesn't mean they don't have that ambition or couldn't evolve a strategy."