At least 20 people have been killed in two villages in northern Nigeria by attackers who slit some residents’ throats and shot others, police say.The gruesome raids by the gang - said to be made up of about 80 people on motorbikes - occurred in the dead of night on Monday in the villages of Dan-Gulbi and Guru, both in Zamfara state"They went door-to-door shooting villagers and in some cases slitting their throats with knives,” the police source said on condition of anonymity."They killed 21 residents. They also moved to the next village where they shot dead two other people. So far 23 villagers were killed in the attack.”They were believed to be in revenge for killings of some of the gang members by villagers and local vigilante groups who grew tired of repeated robberies, the source said. Similar attacks have occurred in the area in the past.A large number of people were also wounded, including a policeman. "Because of the remoteness of the area, police could not deploy in good time and the attackers fled before police teams could reach the area,” the source said."It is clear that the bandits were on a revenge mission.” ‘Vigilante’ responseAl Jazeera’s Yvonne Ndege, reporting from Lagos, said the killings were the latest in a cycle of violence in the area."These two villages were [invaded] by a group of around 80 men, and what we understand the killings were basically revenge on the villagers who had been subjected to robberies and killings and, as a result, formed their own vigilante group,” she said."What we have seen is revenge against those villagers who banded together.”