The death toll in the desert siege of an energy complex in North Africa rose to 37 foreigners, including three Americans, officials said Monday, as new details demonstrated how al Qaeda-linked militants are increasingly equipped to sow terror across the Sahara region.

Algerian officials described a well-organized, heavily armed group of militants who collected advance intelligence on the sprawling natural-gas complex, including its layout and foreign workforce. The attackers commanded an arsenal of missiles, explosives and...