Three crew members are believed to have been kidnapped from a Togolese-flagged bunker tanker off Nigeria.
A possible boarding of the 400-dwt Stelios K (built 1995) by pirates was reported last Monday, 40 nautical miles (74 km) south of Lome in Togo, after it lost contact with its owner.
The incident was confirmed as a rare hijacking two days later.
Pirates had remained on board until the weekend, but security consultancy Ambrey told TradeWinds on Monday that the situation was now over, with three seafarers abducted.
The company is still investigating what went on. Ambrey said it "remains likely this was circumstantial to an unfavourable outcome to a business dispute".
The tanker is now heading to Lagos in Nigeria.
'Seriously strange'
Another source with knowledge of the attack said: "It's a seriously strange case."
The source said the crew appeared to be made up of Greek nationals.
Ambrey said the tanker had been underway at 6.7 knots from Piraeus, Greece, reportedly destined for Lagos, Nigeria.
The tanker was subsequently tracked over the following days as it moved south-west then eastwards, before turning north once again in the Gulf of Guinea.
The ship was said to be 75 nautical miles south of Lagos on Monday.
The motives of the attackers have not yet been disclosed.
"The vessel had just entered the region," Ambrey said. "She had been reflagged from Greek to Togolese within the previous year, and the owner's fleet had otherwise been operating in the Aegean Sea."
Not random
"Given the overall circumstances and general patterns in the region, it is likely that this incident was not based on a random targeting of Stelios K," said Dirk Siebels, senior adviser at security consultancy Risk Intelligence.
"Further investigations into potential links to illicit activities are necessary, but these are unfortunately not likely to lead to conclusive results."
The occupation of the tanker, rather than the immediate abduction of crew, could indicate the criminal gang had sought to either offload or sell the fuel on board, security consultancy Dryad Global said.
The ship is listed as owned by Royal Shipmanagement of the Marshall Islands, which could not be contacted.
Second hijacking this year
This is only the second hijacking in the Gulf of Guinea in 2020, as pirates focus more on kidnappings for ransom.
Dryad said these incidents remain rare due to both their complexity and the high risk of capture by maritime security forces.
The last known hijacking occurred on 15 May when the fishing vessel Hailufeng 11 lost contact and disappeared from satellite tracking.
However, there has been 10 piracy incidents in the region already in November.
Five crew members were kidnapped in the Gulf of Guinea last Monnday from the 1,700-dwt general cargoship AM Delta (built 1992) off Nigeria.
Three days before that, a gang kidnapped 14 Chinese crew members from the 48,127-dwt, Liberian-flagged, heavylift vessel Zhen Hua 7 (built 1998) when it was raided northwest of Sao Tome.
