In pre-dawn darkness, a ­Toyota Land Cruiser skidded off a bridge in North Africa in the spring, plunging into the Niger River. When rescuers arrived, they found the bodies of three U.S. Army commandos — alongside three dead women.

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.

At the same time, the crash in Mali has revealed some details of the commandos’ clandestine activities that apparently had little to do with counterterrorism. The women killed in the wreck were identified as Moroccan prostitutes who had been riding with the soldiers, according to a senior Army official and a U.S. counterterrorism consultant briefed on the incident, both of whom spoke on the condition of anonymity because of the sensitivity of the matter.

The Army’s Criminal Investigation Command, which is conducting a probe of the fatal plunge off the Martyrs Bridge in Bamako, the capital of Mali, said it does not suspect foul play but has “not completely ruled it out.” Other Army officials cited poor road conditions and excessive speed as the likely cause of the 5 a.m. crash.

U.S. officials have revealed few details about the soldiers’ mission or their backgrounds, beyond a brief news release announcing their deaths hours after the accident.

In many countries, including most in Africa, Special Operations forces work openly to distribute humanitarian aid and train local militaries. At times, the civil-affairs assignments can provide credible cover for clandestine counterterrorism units.