Counter-terrorism experts differ as to how the attackers - in several pickup trucks - may have reached In Amenas, but there are several roads and tracks across uninhabited desert from Libya.

CNN: STORY HIGHLIGHTS

Most incidents over the past two years have involved armed groups using Tunisia's southern desert to pass between Algeria and Libya.

NPR: Jihadi Peril Makes Its Way To Tunisia

The crisis began on 16 January when militants attacked two buses carrying foreign workers from the In Amenas gas facility, which lies in the desert in eastern Algeria, not far from the border with Libya.

BBC: Algeria siege dead and survivors flown back to Japan