CAIRO — The only known suspect jailed in connection with the deadly attack on the United States diplomatic mission in Benghazi, Libya, was freed on Tuesday by the Tunisian authorities who had held him. His lawyer said he was released for lack of evidence.

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 was 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.

According to The Associated Press, Mr. Harzi’s lawyer, Anwar Oued-Ali, said his client’s release amounted to “correcting an irregular situation” because no evidence had been presented to substantiate any connection.