The source of much of the concern is Mokhtar Belmokhtar, an Algerian militant who has long been a notorious figure in the Sahel region — a vast area on the southern flank of the Sahara that stretches from Senegal to Chad — and who appears to have become more dangerous even as his ties to Al Qaeda seem to have become more tenuous. Known as Laaouar, or the one-eyed, after losing an eye to shrapnel, Mr. Belmokhtar fought against a Soviet-installed government in Afghanistan…

In January, Mr. Belmokhtar led the attack on a gas plant in Algeria that resulted in the death of 38 civilians, including three Americans. Four months later, his group joined with a Western African terrorist faction — the Movement for Oneness and Jihad in West Africa — to carry out attacks in Niger that killed at least 20 people, the State Department said.

In August, Mr. Belmokhtar’s faction and the West African extremists announced that they were joining to establish yet another group: Al Murabitoun.