The State Department is moving to apply the terrorist designation to two Libyan organizations and one militant believed to have played a role in the deadly attack on the American diplomatic mission in Benghazi in September 2012, senior United States officials said on Wednesday.

The terrorist designations would be the American government’s first formal public accusations of responsibility for the attack, which killed Ambassador J. Christopher Stevens and three other Americans.

Senior United States officials, speaking on the condition of anonymity because the State Department has not yet made the designations public, said they would apply to Ahmed Abu Khattala, a Benghazi militant described by witnesses as having played a role in directing the assault, as well as to an allied group, Ansar al-Shariah of Benghazi, whose fighters were seen participating in the attack. In interviews, Mr. Abu Khattala has denied belonging to Ansar al-Shariah, but the terrorist designation was expected to describe him as a leader of the group. Witnesses said he visited its headquarters the night of the attack. The designation is expected to say that members of the group were involved in it.

The designation was also expected to apply to Ansar al-Shariah of Derna, Libya, which is described as a separate militant Islamist organization, the officials said. The designation was expected to assert that its fighters were also involved in the attack. They may have been identified by witnesses or security camera footage. Derna is a coastal city known as a center of Islamist militancy, a few hours’ drive from Benghazi in eastern Libya.