former secretary of state landed tuesday night at washington reagan national airport after a first-class flight from boston .
clinton stared ahead wordlessly , walked and did n't acknowledge questions as daily mail online asked whether she had made mistakes in benghazi .
deadly attacks , carried out by an affiliate of al qaeda , claimed the lives of u.s. ambassador to libya chris stevens and three other us personnel .
special congressional committee investigating circumstances before and after the attacks ; its report will come out during 2016 presidential campaign .
questions remain about clinton 's alleged failure to protect her diplomatic outpost , and the reasons she cited for the debacle after four flag-draped caskets came home .
u.s. intelligence knew at the time that benghazi personnel were aware they were under attack , but clinton and other obama administration officials blamed an out-of-control protest linked to an anti-islam youtube video .