PARIS (AP) — Four aid workers for a French Christian charity disappeared this week in Baghdad at at time of heightened tension in Iraq, the organization said Friday,

The charity, SOS Chrétiens d’Orient, said the four — three French citizens and an Iraqi — failed to show up for a scheduled meeting Tuesday afternoon and have not been heard from since. All four had prior experience in crisis zones and were staying at a hotel that regularly hosts international guests. They were in Baghdad primarily for administrative reasons, to renew visas and register the charity with the Iraqi government.

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The group said there have been no ransom demands. French and Iraqi authorities are cooperating in the investigation.

The four went missing during a time of heightened tensions in Iraq after a U.S. drone strike on Baghdad airport that killed Iranian Gen. Qassem Soleimani and a senior Iraqi militia commander, Abu Mahdi al-Muhandis. The attack has drawn anger from Iraqi officials from across the political divide and lead to a Jan. 5 non-binding parliamentary resolution to oust U.S. troops from the country.

Iran-backed militia groups have also sworn to avenge the killings.