US officials have implicated a former Guantanamo Bay prisoner in the 2012 attack on the US mission in the eastern Libyan city of Benghazi, The Washington Post reported Tuesday. Militiamen led by Abu Sufian bin Qumu, chief of extremist group Ansar al-Sharia in Derna city, took part in the attack that killed US ambassador Christopher Stevens and three other Americans, according to the daily. It said US officials plan to designate three branches of the group as a foreign terrorism organization in Derna, Benghazi and Libya’s neighbor to the west Tunisia in the coming days. Qumu and two others — militia leaders Ahmed Abu Khattala and Seif Allah bin Hassine — will also be identified as “specially designated global terrorists,” the Post said, citing US officials familiar with the plans. The move aims to pressure the group and curtail support for terror activities, as it allows US authorities to freeze the group’s and the men’s financial assets and bars US citizens and companies from doing business with them. Witnesses told US officials that Qumu’s men had made the hours-long drive to Benghazi from Derna before the September 11, 2012 attack, the Post said, though it is unclear whether they were there as part of a pre-planned attack. US officials had sought to get a hold of Khattala in Benghazi, but dropped the plans out of fear that such a move by Americans could foment unrest and destabilize the Libyan government, the Post said. Khattala has denied any role in the Benghazi raid. According to the report, US officials are also trying to determine whether any of the people involved in the Benghazi assault were involved in the killing of American schoolteacher Ronnie Smith, who was gunned down while jogging in the restive city last month.