"We are deeply troubled by the lack of transparency in the legal process that resulted in the release of the only individual remaining in custody," State Department spokesman Ned Price said in a statement.
Sudanese authorities on Monday freed Abdelraouf Abu Zaid, who was convicted over the killing, with his lawyer saying it was a court decision in line with a 2020 compensation package by Sudan to Washington for past terrorism.
Price said the United States was seeking clarity on the release of Abu Zaid and was offering $5 million for information leading to the arrest or conviction of two other people suspected in the 2008 killings.
He said it was "inaccurate" that the United States had agreed to the release as part of the 2020 deal, which removed Sudan from a blacklist of state sponsors of terrorism dating from the dictatorship of Omar al-Bashir.
As part of the deal brokered by then-PM Abdalla Hamdok, impoverished Sudan paid $335 million to American survivors and families of victims killed in past attacks.
Hamdok, a civilian heading a transitional government, was seeking to reintegrate Sudan into the international community but he was ousted the following year by the military, setting back relations with the United States which froze $700 million in economic support.
