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.

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.