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The release dramatized the negligible progress in any investigation into the attack, which killed Ambassador J. Christopher Stevens and three other Americans on Sept. 11 last year. The feebleness of Libya's transitional government since the fall of Col. Muammar el-Qaddafi has stymied any progress despite what Benghazi residents describe as an abundance of leads.

The man released Tuesday, Ali Harzi, 28, was detained in October at an airport in Turkey and deported to his home country on suspicion of involvement in the attack. F.B.I. agents investigating the Benghazi attack and another on the United States Embassy in Tunis reportedly questioned Mr. Harzi in December. But it is not known if the questioning yielded relevant information.

His lawyer, Anwar Oued-Ali, told The Associated Press that the release on Tuesday amounted to "correcting an irregular situation" because no evidence was presented to substantiate any connection. The attack was filmed by surveillance cameras and observed by large crowds of witnesses. Several Libyan and American guards who tried to defend the mission and saw the attackers survived the assault. And several Libyan witnesses have identified at least one local Islamist militant seen leading the attack, Ahmed Abu Khattala, to journalists and United States law enforcement agents.

But Mr. Abu Khattala remains at large in Benghazi. This week, in fact, he survived a vigilante assassination attempt; a car bomb reportedly killed the would-be assassin. Libya's interim government has not disclosed the names of any other suspects or anyone else investigators might have held or even questioned in connection with the attack. And the Benghazi militias who provide the main law enforcement in the city have so far declined to take any action against Mr. Abu Khattala or the Islamist group most widely linked to the attack, Ansar al-Shariah, citing insufficient evidence.

Egyptian authorities in Cairo have said they detained a militant reported to have links to the Benghazi attacks, but United States officials have played down that connection and suggested that the Egyptians had many causes to arrest the militant.