They're rogue armed men in military fatigues who sweep through the cities and towns of northeastern Nigeria, killing civilians or taking them away, never to be seen again.
If the description matches the Nigerian extremist group, Boko Haram, think again. The alleged killers described in a report Wednesday by the London-based rights group Amnesty International are Nigerian soldiers.
Amnesty said abuses by Nigeria's military caused the deaths of more than 8,000 civilians in the battle against Boko Haram. They include about 1,200 who were the victims of extrajudicial executions since February 2012, and 7,000 who died of mistreatment in detention since March 2011, according to the report.
Some starved to death. Others died of thirst or suffocated to death in poorly ventilated cells jammed with prisoners. And some died when soldiers fumigated the cells with mosquito poison, the report said.
Amnesty called for the investigation of senior commanders for possible war crimes, including murder, torture and enforced disappearances.
"The hundreds of unidentified bodies, the evidence of mass graves and the harrowing stories of starvation and abuse coming out of the country's military barracks demand nothing less than an urgent investigation and for those responsible to be brought to justice," Amnesty International Secretary-General Salil Shetty said in a statement Wednesday.
The report was the product of a four-year investigation involving hundreds of leaked military documents and interviews with more than 400 witnesses, victims, doctors, senior military personnel and others, Amnesty said.
According to the report, "senior officials of the Nigerian military had full knowledge of the arbitrary detentions and high rates of deaths and failure to take action to stop these human rights violations."
The most notorious detention facility is at Giwa barracks in Maiduguri, according to the report. A rough pen sketch by a former prisoner labeled one corner "corpses" -- the place where the dead were piled up.
"The soldiers said: 'Welcome to your die house. Welcome to your place of death,'" said a man who spent four months at the barracks.
He said he was chained to another man and crammed into a 25-by-25-foot cell with about 400 detainees.
"They started to die after three days," he told Amnesty. Only 11 out of the 121 men and boys with whom he was arrested survived the ordeal.
Another former detainee described drinking urine to survive because no water was provided.
"But even the urine at times you cannot get," he said. "Any time we were denied water for two days, 300 died ... and whenever someone died, we were happy because of the extra space."
A high-ranking military officer, who was not identified in the report, confirmed the horrifying conditions at Giwa.
"People were not strong enough to stand," he said. "They are deliberately starved. You have massive deaths."
At least 20,000 men and boys, some as young as 9, have been arrested in the northeast on suspicion of being Boko Haram members since 2009, often swept up randomly in raids, without adequate investigation or evidence against them, according to Amnesty.
They include a 25-year-old carpenter, who said he was detained in a Nigerian military raid in Maiduguri and held in a small cell with dozens of other men.
"Space in the cell was so limited, they would have to take turns to sit down," the report said. "With restricted ventilation, many died of suffocation. Meals consisted of a portion of rice that would fit in his hand, once or twice a day, and water was seen as a luxury."
On the worst day, he saw 80 people die, the report said. Just four of the 19 people he was arrested with survived.
The man escaped when Boko Haram attacked the barracks in March last year, freeing hundreds of inmates. Many others were killed when Nigerian soldiers and air force planes fired at them as they fled.
A senior military official gave Amnesty a list of 683 detainees who had died at the Giwa barracks over a five-month period beginning in October 2012. The official estimated that close to 5,000 people had died there since the beginning of 2013.
Amnesty researchers documented some of the emaciated bodies brought to local mortuaries from the barracks. More than 1,400 corpses were delivered to a single Maiduguri morgue in June 2013, the report said.
The report also detailed allegations of abuse at a military facility in Potiskum, in Yobe state, where prisoners were reportedly kept in a deep hole for days.
"They poured cold water on us, and at other times they burn polythene and drop the hot melting polythene on our backs. I spent over three days in this hole," one prisoner said.
Amnesty said it had obtained 90 videos showing members of the security forces and allied militiamen committing abuses. The report contained images of Nigerian soldiers beating men with large sticks, or walking on the backs of prisoners lying on their stomachs.
Nigeria's military is under intense pressure over its failure to swiftly contain the Boko Haram insurgency. A vast swath of territory fell to Boko Haram last year after soldiers ran away rather than defend towns from the group.
With the help of neighboring armies and foreign mercenaries, Nigeria's military has managed to drive Boko Haram back into a wedge of dense forest in recent months.
Nigeria's new president, Muhammadu Buhari, acknowledged in his inauguration address last week that abuses by the security forces -- including the extrajudicial killing of Boko Haram's founder, Mohammed Yusuf -- may have contributed to the group's growth, a sharp departure from the previous administration, which usually swept such allegations under the carpet.
(Amnesty said it wrote 57 letters to Nigerian authorities, demanding investigations and action to stem military abuses, without results.)
"Boko Haram is a typical example of small fires causing large fires," Buhari said, as the nation's top military commanders looked on. "An eccentric and unorthodox preacher with a tiny following was given posthumous fame and following by his extra judicial murder at the hands of the police. Since then through official bungling, negligence, complacency or collusion, Boko Haram became a terrifying force taking tens of thousands of lives and capturing several towns and villages covering swathes of Nigerian sovereign territory."
Buhari pledged to rewrite Nigeria's military code and ensure that abuses are fully investigated.
The reports of abuse by Nigerian soldiers date back years and have at times been the subject of extensive coverage in the national media.
After Boko Haram launched its 2009 uprising in Maiduguri, video emerged of military officers rounding up suspects, making them lie down in the streets and shooting them on the spot.
"Shoot him in the chest, not the head," one security official told a subordinate in a video as two men on crutches were forced to the ground. "I want his hat."
Seventeen police officers were arrested after a national outcry, one of the rare cases when security officials have faced charges for extrajudicial killings.