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.

In November, the official in charge of Benghazi security was assassinated, and on Sunday the Libyan government announced that the investigator sent to look into his death was himself kidnapped.

"You've had a long series of police and militia leaders kidnapped and killed in Benghazi," noted Lawrence, explaining why neither the Libyans nor the U.S. has been able to get far in the investigation. "Benghazi is certainly a very bad place to be a police officer."

For their part, Libyan officials have remained silent on the course of the investigation in the ambassador's death, saying only that it is ongoing.

Harzi was one of very few people in custody in relation to the attack, along with Jamal Abu Ahmad, a member of Islamic Jihad arrested in Egypt, according to U.S. officials.

Oued-Ali, the lawyer, described the release as "correcting an irregular situation" because authorities never had any real evidence.

Harzi is still officially charged with membership in a terrorist organization - a charge punishable by six to 12 years in prison. In December, FBI officials questioned Harzi for three hours in the presence of a Tunisian judge.

The FBI has not commented on the results of the questioning, but Harzi's lawyer said they just asked if his client had any information about the attacks on the Benghazi mission as well as the assault on the U.S. embassy in Tunis three days later.

U.S. intelligence has blamed the Benghazi attack on militants who are members of a number of different groups. They range from the local Libyan militia Ansar al-Shariah, whose members were seen at the U.S. consulate during the attack, to militants with links to al-Qaida in the Islamic Maghreb - al-Qaida's leading representative in the African region. The consulate's cameras captured many of the faces of armed men in a mob, and some have been questioned, but most remain free.

In a recent TV interview, Harzi's father, Tahar, said his son was just working in Libya in construction supporting his family.

Both Ali Harzi and his brother Brahim have had brushes with the law before, however. In 2005, the two were sentenced to 30 months in prison for having contact with another brother, Tarek, who fought against coalition forces in Iraq, according to lawyer Oued-Ali.

Tarek recently escaped from an Iraqi prison in the city of Tikrit and is at large, added the lawyer.

The father acknowledged in the interview that he had encouraged his sons to take up "jihad in the cause of God."