Officials in Switzerland have announced the death a month ago of a Swiss woman held hostage in Mali.
Foreign affairs chief Ignazio Cassis condemned the killing, saying officials were trying to locate her remains.
They learned of the death after French charity worker Sophie Pétronin alerted the French authorities.
Ms Pétronin, 75, was seized in December 2016 and freed this week. She was the last French citizen to be held hostage anywhere in the world.
Two Italian nationals and the ex-Malian opposition leader Soumaïla Cissé were also released this week.
The release was part of a prisoner swap for more than 100 jihadists, believed to be affiliated to al-Qaeda.
The Malian presidency has not revealed how it was able to free the hostages.
Malian and international armed forces have been struggling to contain a jihadist insurgency in the north of country that first emerged in 2012.
What has Switzerland said?
In a statement, the Swiss foreign ministry said the woman - who has not been named - was "apparently killed by kidnappers... about a month ago".
Information about her killing "was obtained by the French authorities from the recently released French hostage" Ms Pétronin, it said.
It added that officials "are making every effort to find out more about the circumstances of the killing and the whereabouts of the remains", and that the ministry has demanded that the body be handed over.
Mr Cassis added: "It was with great sadness that I learned of the death of our fellow citizen. I condemn this cruel act and express my deepest sympathy to the relatives."
What's the latest with the hostage release?
Ms Pétronin and Mr Cissé, who was abducted in March while campaigning for parliamentary elections, were taken to the capital, Bamako, in a military plane along with the two Italians, Mali's presidency announced on Thursday.
The two Italian hostages released were named as Pierluigi Maccalli, a missionary priest who was kidnapped in 2018, and Nicola Chiacchio, who was believed to have been captured while travelling in the region as a tourist.
Earlier on Thursday, the Malian authorities announced the release of a dozen political and military figures arrested during the coup.
In August, Malian President Ibrahim Boubacar Keïta was overthrown by a military junta and little is known of what happened to talks to secure a prisoner swap after the coup.
What's behind Mali's Islamist insurgency?
A separatist rebellion in the north of the country that began in 2011 created the conditions for militant jihadists to take control of parts of the region.
In 2013, a French-led force helped seize back territory, but a network of jihadist groups remained active and they were able to carry out attacks and kidnappings.
The Malian government has not been able to regain full control of the north.
France continues to support forces in Mali and in other parts of the Sahel region in their fight against the militants.