Employees recognised the corpse of their erstwhile colleague, who had worked as a driver for one of the companies operating at In Amenas, a source told the AFP news agency.
Japanese photo agency has published the first pictures to emerge from the In Amenas incident. View them here.
It says they were taken by an Algerian worker on his mobile phone.
He had apparently left his job a year previously.
Two more of the attackers are reported to have been known as “the Canadians” and had joint Algerian-Canadian nationality.
There were also three Algerians and men from five other countries – Egypt, Mauritania, Mali, Niger and 11 from Tunisia, officials said.
The leader of the attack, Mohamed Amine Bencheneb, who was killed by the military, was behind the 2011 kidnapping of two Spaniards, who have since been released, in south-west Algeria.
Japanese Senior Vice Foreign Minister Shunichi Suzuki arrived in Algiers on Wednesday with a brief to find out how seven Japanese citizens came to be killed in the Algerian operation to end the Islamist take-over of the plant.
He came aboard a government jet that will repatriate the bodies of the victims.
Three other Japanese are still missing and Suzuki had a letter from Prime Minister Shinzo Abe to Algerian President Abdelaziz Bouteflika asking for efforts to be made to find out what has happened to them.
It has been revealed that on Friday Japan, along with Britain, the US and other countries whose nationals were at the plant, made a joint formal representation to Algeria about the conduct of the operation and the danger to captives’ lives.
The attackers said that the assault, in which 37 foreigners, an Algerian worker and 29 jihadis are said to have died, was in reprisal for the French intervention in Mali.