A Swiss woman kidnapped four years ago in Mali has been killed by her captors, according to the Swiss foreign ministry.

“Friday afternoon the French authorities informed the Swiss authorities that the Swiss hostage in Mali is dead, says a foreign ministry press release. “She was apparently killed by kidnappers of the Islamist terrorist organization Jama’at Nasr al-Islam wal Muslim (JNIM) about a month ago.”

The press release does not name the hostage, but Swiss news agency Keystone-SDA reports that she was Béatrice Stöckli, a missionary from Basel who worked in Timbuktu.

Swiss foreign minister Ignazio Cassis expressed “great sadness” at news of her death. “I condemn this cruel act and express my deepest sympathy to the relatives,” he said.

The exact circumstances of the killing are currently still unclear. News of the death “was obtained by the French authorities from the recently released French hostage”, says the ministry press release. Franco-Swiss aid worker Sophie Pétronin, a Malian opposition politician and two Italians were released in Bamako, Mali, on Thursday night, ending their lengthy captivity in the hands of Islamist insurgents.

The Swiss authorities say they are “making every effort to find out more about the circumstances of the killing and the whereabouts of the remains” and would also approach the transition government in Mali to this end