WASHINGTON — The attack in Benghazi, Libya, that killed Ambassador J. Christopher Stevens and three other Americans has dealt the Central Intelligence Agency a major setback in its intelligence-gathering efforts at a time of increasing instability in the North African nation.

Among the more than two dozen American personnel evacuated from the city after the assault on the American mission and a nearby annex were about a dozen C.I.A. operatives and contractors, who played a crucial role in conducting surveillance and collecting information on an array of armed militant groups in and around the city.

“It’s a catastrophic intelligence loss,” said one American official who has served in Libya and who spoke on the condition of anonymity because the F.B.I. is still investigating the attack. “We got our eyes poked out.”

The C.I.A.’s surveillance targets in Benghazi and eastern Libya include Ansar al-Sharia, a militia that some have blamed for the attack, as well as suspected members of Al Qaeda’s affiliate in North Africa, known as Al Qaeda in the Islamic Maghreb.