Six policemen have been killed in Nigeria's oil capital, Port Harcourt.
Their bodies, which had bullet wounds in them, were found floating in a river in the city. Two other members of the marine police patrol survived.
The police have not said who they suspect is behind the killings and no group has claimed responsibility.
Militants have staged a series of attacks in the region recently to push their demands that local people should benefit more from the oil wealth.
Last week, three foreign oil workers were briefly kidnapped in the city and a US man was shot dead.
An upsurge in attacks on foreign oil interests has cut the country's oil production by 20%.
Last month, the Movement for the Emancipation of the Niger Delta (Mend) renewed its threat to continue oil installation attacks, but have denied responsibility for the last week's.
It said the offer of thousands more jobs and a new motorway from President Olusegun Obasanjo did not address its demands for more local control of oil wealth and demilitarisation.
The Mend group has claimed responsibility for two car bomb attacks in recent weeks, one of which was a warning against Chinese expansion in the region, it said.