Tripoli/Washington: Libyan authorities have detained a man investigators believe could be an important witness or suspect in the attacks on US outposts in Benghazi, Libya last September, according to people familiar with the matter.

The man, a Libyan national identified as Faraj Al Chalabi, fled to Pakistan after the attacks and only recently returned to Libya, said the sources, who include people in the United States and Libya close to the ongoing investigations. One Libyan security source said he was from Eastern Libya.

The US government is aware of Al Chalabi’s detention and there are signs American investigators may have been able to pose questions to him, according to the sources. It is not clear whether those questions were posed in person or through Libyan authorities.

Precisely what role the detained man played in the September 11, 2012 attacks is unclear.

US sources in Washington said they did not believe he was a principal instigator or a front-line leader of the attacks on a poorly guarded temporary US diplomatic mission compound and a more fortified CIA compound nearby.

US Ambassador Christopher Stevens, two CIA security officers and another US diplomat were killed in the attacks.

The Benghazi incident became a major headache for President Barack Obama’s administration just months before the November presidential election. Republicans assailed the Democratic president partly over security lapses, but also over the administration’s conflicting early accounts of what happened.

The Federal Bureau of Investigation has been investigating the case and its agents have visited Libya. So far, no individuals are known to be facing criminal charges in connection with the attacks.

At a hearing of the Senate Intelligence Committee on Tuesday, FBI Director Robert Mueller said FBI investigators had been on the ground in Tripoli and elsewhere, and he believed the inquiry will “prove to be fruitful.” He did not elaborate.

An FBI spokesman said on Wednesday Mueller’s remarks “speak for themselves” and “Benghazi remains an ongoing investigation.”

A senior Libyan official confirmed an arrest had recently been made. “There is progress in the investigation. We hope it is going in the right way,” the official said, without providing more details.

At least two other men suspected of being connected to the attacks were detained by authorities outside Libya at one time or another, said one US source familiar with the investigation.

In one case, Ali Harzi, a Tunisian, was picked up in Turkey and returned by authorities to Tunisia, where he was held for a time and FBI agents were allowed to question him. His lawyer later said Harzi had been conditionally set free due to a lack of evidence.

In another case, Mohammad Jamal, leader of a militant faction based in Egypt, was detained and held by Egyptian authorities. A US source said Jamal, an alleged former operative of the Egyptian Islamic Jihad, was still believed to be in custody.

It is not clear if US authorities have been able to question him, however, and US officials have said the role of Jamal in the Benghazi attacks was also unclear. However, US officials have said they believe some of Jamal’s followers likely were involved in the attacks.