The man on the line spoke Arabic, telling embassy officials that Stevens had been taken to a hospital in Benghazi.
Officials could not determine what hospital Stevens was taken to.
Some wondered if the phone call was a trick from militants who wanted to lure U.S. officers to their death.
A Libyan official was sent to Benghazi Medical Center. He said Stevens was there.
Hospital staff said six civilians brought Stevens to the emergency room about 1:15 a.m.
Even though the ambassador showed no signs of life, doctors worked to revive him for 45 minutes.
It was too late.