Khost Province has suffered from multiple attacks, including suicide bombings near the main U.S. military camp called Solerno. In May and June, attacks in Khost City and outside Solerno killed at least 21 civilians, most of them Afghan day laborers.
But the terror attack was directed at a non-military camp and appears to be the deadliest against U.S. civilians since the start of the war. In August, 16 civilians died in a helicopter crash in Kandahar, but that was the result of mechanical failure.
ABC News' Kirit Radia contributed to this report.