A man under arrest in Tunisia is being investigated by the US in connection with the September 11 attacks on US diplomatic facilities in Benghazi, Libya, US government sources say.
The 28-year-old suspect, identified earlier this week by the Daily Beast news website as Ali Ani al-Harzi, was one of two Tunisians detained by authorities in Turkey early in October.
News reports at the time of their detention said that the two men were stopped at an Istanbul airport as they tried to enter the country using false passports. The Turkish embassy in Washington declined to comment.
US government sources have said that only one of the men, now understood to be al-Harzi, was under investigation in connection with the Benghazi attacks, in which Christopher Stevens, the US ambassador to Libya, and three other American officials were killed.
One of the sources said that Turkish authorities subsequently sent al-Harzi back to Tunisia.
Al Jazeera's Hoda Abdel-Hamid reports on the Libyan investigation in the September 11 Benghazi attack.
An official at the Tunisian justice ministry confirmed a man had been arrested, but denied he was linked to the killing of Stevens.
"The arrested person was deported by Turkey last month, but he has nothing to do with the killing of the US ambassador," he told the Reuters news agency without giving more details.
Another US government source said US investigators believed that al-Harzi "had something to do with [the Benghazi violence] for sure".
The sources said, however, that at this point al-Harzi was not being investigated as a leader or organiser of the Benghazi attacks, but rather as a possible participant.
The Daily Beast reported that shortly after the Benghazi attacks began, al-Harzi posted an update on an unspecified social media site about the fighting.
The Daily Beast said this was one of the first clues that US intelligence agencies acquired as to who might be behind the violence.
The Daily Beast said al-Harzi was on his way to Syria when he was detained in Turkey at the behest of US authorities, and that he was affiliated with an armed group in North Africa.
Neither the website nor US government sources identified the group with which he allegedly is associated.
Separately, a Tunisian court on Wednesday sentenced a leader of the Ansar al-Sharia group to one year in prison for inciting an attack on the US embassy in Tunis last month in which four people were killed.
The conviction of Abu Ayub was the first in connection with the attack - launched to protest against an anti-Islamic film made in California - and is likely to please the US which has been urging the authorities to jail those responsible.
"The judge decided to jail him on charges of inciting violence ... This decision is unfair ... we will go to appeal," Rafik Ghak, a lawyer for Abu Ayub, said.
The authorities arrested 144 people, including two prominent leaders of Ansar al-Sharia after the embassy attack. Seif Allah Ben Hassine, the leader of the group, has urged the government to release them.
Benahssine on Tuesday accused the country's government of being a puppet of the US and un-Islamic.
The US this month requested Tunisia bring the embassy attackers to trial, pledging to continue its support for the democratic transition in Tunisia.
Tunisia expects the US to guarantee around a fifth of its $2.2-2.5bn borrowing needs next year to help its economy recover from last year's revolution, the country's minister of international co-operation, Riadh Betaib, said last month.