In the hours after the Benghazi attack, the American official said, spy agencies intercepted electronic communications from Ansar al-Shariah fighters bragging to an operative with Al Qaeda in the Islamic Maghreb, an Algerian insurgency that has made itself a namesake of the global terrorist group founded by Bin Laden. Another intercept captured cellphone conversations by militants on the grounds of the smoldering American Mission in Benghazi that suggested links to, or sympathies for, the regional Qaeda group.

In Benghazi, Ansar al-Shariah’s role in the attack has been an open secret since it began. The group’s leaders had boasted of their ability to flatten the United States Mission compound. Witnesses saw trucks emblazoned with the logo of their brigade at the scene, fighters who assaulted the compound acknowledged their affiliation with the group and witnesses saw their faces. Some Libyan guards at the compound saw them close up, and injured attackers were then treated at local hospitals.

Although in the immediate aftermath of the attack Libyan officials issued conflicting and unverifiable reports about arrests, the Libyan government has never identified anyone detained, and in recent days officials have declined repeated requests to provide any details.

Members of the new national congress, speaking on the condition of anonymity because of the continuing investigation, acknowledged with frustration on Tuesday that Ansar al-Shariah members had not been questioned and remain at large.

Mustafa el-Sagizli, a senior official in the transitional government and an officer in one of Benghazi’s main militias, the February 17 Brigade, said the government had detained a few looters. But he said he did not believe anyone had been detained in connection with the attack itself.

“It was a mess,” he said, “and it was hard to tell who did it.”

Leaders of the large militias that provide the only law enforcement in Benghazi all say they are awaiting further evidence or formal orders before any move to round up or bring in members of Ansar al-Shariah. Privately, militia leaders complain that detaining Ansar al-Sharia’s leaders and fighters could mean a bloody clash within their own close-knit community.

In an interview on Tuesday, Ibrahim el-Sharkasi, a top official of the Interior Ministry, said he had no knowledge of any detentions or interrogations. He insisted that a special judge had been appointed to lead the investigation. But he said he could not name the judge. And there have been no reports of such an appointment.