TUNIS, Tunisia – Tunisian authorities released one of the only men in custody for alleged links to September’s attack on the U.S. diplomatic mission in the Libyan city of Benghazi, the latest blow to an investigation that has limped along for months.

Armed groups assaulted the lightly guarded mission on Sept. 11 and killed U.S. Ambassador Christopher Stevens and three other Americans, but despite U.S. promises there has been little news of progress so far in bringing the perpetrators to justice.

Ali Harzi, a 26-year-old Tunisian extradited from Turkey in October, was one of the only people actually detained over the attack, and at the time Tunisian authorities said they “strongly suspected” he was involved.

On Tuesday, however, his lawyer Anwar Oued-Ali said the presiding judge had “conditionally freed” Harzi the night before for lack of evidence. He must remain in the Tunis area to be available for any further questioning.

William Lawrence, the North Africa analyst for the International Crisis Group, said while it was very possible that Harzi might have been involved with extremist groups in Benghazi, it was impossible to tell without more efforts from the Libyans.

“If there had been a better investigation in Benghazi, this guy’s role in the whole thing would have been a lot clearer,” he said.

“The fundamental issue is that the Libyans aren’t prioritizing this.”

Already back in December, U.S. officials were lamenting the lack of cooperation with the governments of the region, particularly Libya, in their ongoing investigation into the attack, saying most of the suspects remain free.

Mounting a coherent investigation is difficult for Libyan government – especially in Benghazi – because authorities rely on the militias who fought former leader Moammar Gadhafi and it is often difficult to draw clear lines between those providing security and those causing the instability.