What the men were doing in the impoverished country of Mali, and why they were still there a month after the United States suspended military relations with its government, is at the crux of a mystery that officials have not fully explained even 10 weeks later.

At the very least, the April 20 accident exposed a team of Special Operations forces that had been working for months in Mali, a Saharan country racked by civil war and a rising Islamist insurgency. More broadly, the crash has provided a rare glimpse of elite U.S. commando units in North Africa, where they have been secretly engaged in counterterrorism actions against al-Qaeda affiliates.

The Obama administration has not publicly acknowledged the existence of the missions, although it has spoken in general about plans to rely on Special Operations forces as a cornerstone of its global counterterrorism strategy. In recent years, the Pentagon has swelled the ranks and resources of the Special Operations Command, which includes such units as the Navy SEALs and the Army’s Delta Force, even as the overall number of U.S. troops is shrinking.