U.S. officials suspect that a former Guantanamo Bay detainee played a role in the attack on the American diplomatic compound in Benghazi, Libya, and are planning to designate the group he leads as a foreign terrorist organization, according to officials familiar with the plans. Militiamen under the command of Abu Sufian bin Qumu, the leader of Ansar al-Sharia in the Libyan city of Darnah, participated in the attack that killed U.S. Ambassador J. Christopher Stevens and three other Americans, U.S. officials said. Witnesses have told American officials that Qumu’s men were in Benghazi before the attack on Sept. 11, 2012,...