Those eight Americans initially planned to leave the airport with Mr. Fathi and a handful of Libyan militiamen in four vehicles, two Toyota Land Cruisers followed by two Kia sedans. But when they learned of the Americans’ arrival, local Libyan security forces insisted on sending 16 more vehicles of fighters, Mr. Obeidi said. “I told them not to be too close to us so when we get to the place we don’t create a scene,” he said.

But the attackers had evidently found it, perhaps by following the vehicle leaving the compound.