CAIRO — Libyan authorities have singled out Ahmed Abu Khattala, a leader of the Benghazi-based Islamist group Ansar al-Sharia, as a commander in the attack that killed the US ambassador to Libya, J. Christopher Stevens, last month, Libyans involved in the investigation said Wednesday.

Witnesses at the scene of the attack on the US Mission in Benghazi have said they saw Abu Khattala leading the assault, and his involvement is the latest link between the attack and his brigade, Ansar al-Sharia, a puritanical militant group that wants to advance Islamic law in Libya.