For the last eight years they had followed well-publicized routes through Morocco, Mauritania, Senegal and Gambia. This time, as the volunteers traversed the stretch of sand-swept pavement that is Mauritania’s main highway, a group of armed men lay in wait.

Rather than solidarity, it seems, the gunmen sought hostages.

Members of the convoy heard gunshots at about 8 p.m. on Sunday, Nov. 29, they told reporters. Those in front raced back to the caravan’s last car. The Land Rover stood abandoned in the dark, doors gaping open. Money and equipment remained inside. The passengers had vanished.

Spanish authorities have blamed the kidnapping on an organization called “Al Qaeda in the Islamic Maghreb.” It is the same group authorities believe abducted a French man from his hotel in northern Mali just a few days earlier.