A Nigerian court has dropped arms trafficking charges against seven Russian sailors a year after they were arrested.
The remaining eight of those charged were acquitted earlier this year.
The BBC's Will Ross in Lagos says it is not clear why it has taken so long for all 15 sailors to be cleared.
He says the case has focused attention on the increasingly dangerous waters off West Africa's coast and the spread of illegal arms in the region.
The navy had said 14 assault rifles and several thousand rounds of ammunition were found on board MV Myre Seadiver last October.
Our correspondent says that during the trial, it never became clear why the Russian sailors had been carrying the weapons.
Piracy has become a major problem in the Gulf of Guinea, and it is possible that the Russian men had the weapons for their own protection or that the ship was being used as a floating arsenal to safeguard other vessels, he says.
Carrying weapons at sea can lead to legal problems.
In 2010 when piracy was at its peak off the east coast of Africa, the Eritrean government accused several British security guards of terrorism and sabotage after finding them with weapons inside that country's territorial waters.
In West Africa, oil tankers are particularly at risk from attack.
Many companies now pay private security firms or even the Nigerian military for protection.
The fact that ships have even been attacked as they are moored outside Lagos port has led to some suspicion that members of the military are involved in the lucrative racket of stealing fuel, our correspondent says.