A email purportedly from the Movement for the Emancipation of the Niger Delta (Mend) said it was responsible for the ambush in southern Nigeria on Friday.
Last week another email said Mend would resume attacks after its leader, Henry Okah, was jailed in South Africa.
Police denied Friday’s attack had anything to do with the threat.
They said it involved a dispute over amnesty payments.
Thousands of former militants are supposed to be receiving a monthly salary as part of the amnesty agreement.
However, while some former commanders have grown extremely rich out of the deal, some of the junior militants have not received what they are owed, the BBC‘s Will Ross reports from Lagos.
The 12 police officers were reported missing, presumed dead, after gunmen attacked a police boat in the creeks of the Niger Delta, officials said.
The boat carrying some 50 police officers was on its way to a funeral when it developed engine trouble in one of the creeks.
“The officers became soft target for some hoodlums, who we have confirmed were part of a militant group that was supposed to be enjoying an amnesty,” police commissioner Kingsley Omire said.
He said the group was linked to a former Mend militant, and that the assailants alleged that the ex-militant had not been properly distributing amnesty payments.
All but the 12 officers reported missing were now safe, he added.
Mend had been fighting to gain a greater share of the oil wealth from its part of southern Nigeria, but had been inactive since a 2009 amnesty was put in place.
Okah, its leader, was sentences to 24 years in prison last month for masterminding bomb attacks in the capital, Abuja, in 2010.
A resumption of violence in the delta could hit Nigeria’s vital oil industry.
This week, President Goodluck Jonathan ordered a committee to look into the possibility of making a peace deal with Islamist militants in northern Nigeria, where they have carried out numerous bombings and shootings in recent years.