A speedy, effective response by newly trained Libyan security guards to a small bombing outside the American diplomatic mission in Benghazi in June may have led United States officials to underestimate the security threat to personnel there, according to counterterrorism and State Department officials, even as threat warnings grew in the weeks before the recent attack that killed Ambassador J. Christopher Stevens and three other Americans.

The guards’ aggressive action in June came after the mission’s defenses and training were strengthened at the recommendation of a small team of Special Forces soldiers who augmented the mission’s security force for several weeks in April while assessing the compound’s vulnerabilities, American officials said.

“That the local security did so well back in June probably gave us a false sense of security,” said one American official who has served in Libya, and who spoke on condition of anonymity because the F.B.I. is investigating the attack. “We may have fooled ourselves.”