Among the dead were five men who had been pardoned for past militant activity under a government amnesty program, The Guardian newspaper in Nigeria reported Sunday.
The men were killed Saturday night after being lured to a creek in the village of Lorbia in the southern coastal state of Bayelsa, officials said.
Accounts differed on the reason for the killings.
One report said the deaths occurred in a gunbattle between armed gangs. Another said mercenaries hired by a militant group were responsible.
Local residents said the suspects were believed to be involved in the recent killings of 12 police officers.
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