Tunisian authorities have unconditionally released the only suspect they held in custody over the deadly attack on the US consulate in the Libyan city of Benghazi, his lawyer says.

Anwar Oued-Ali said on Tuesday his client, Ali Harzi, was freed on Monday for lack of incriminating evidence but had to remain in the greater Tunis area in case the court needed him, according to the Associated Press.

The release of Harzi, a Tunisian, represents a blow to the investigation of the attack on the consulate, which killed the US ambassador and three other Americans.

The attack coincided with the 11th anniversary of 9/11 assault on the World Trade Centre in New York and the Pentagon.

The investigation in Libya itself has been hamstrung by the weak power of the central government in the face of the powerful militias, some of whom may have been involved in the attack.

According to Associated Press news agency, Harzi had previously been interrogated by FBI officials in the presence of a Tunisian judge in December.

He was originally detained in Turkey and in October was extradited to Tunisia, where authorities had said he was "strongly suspected" of being involved in the attack.

His lawyer had always maintained there was no evidence linking him to the attack.

Hillary Clinton, US secretary of state, admitted last October that she assumed "responsibility" for what happened on September 11.

She said Barack Obama, the president, and Joe Biden, his vice president, "wouldn't be knowledgeable about specific decisions that are made by security professionals".

A report by the Accountability Review Board said in December "systematic failures" and "leadership and management deficiencies at senior levels" at the state department led to the attack.