But authorities don't know where Ahmed Abu Khattalah is, they told The New York Times and The Wall Street Journal.
Government-loyal militias that maintain security until effective police forces are established told the Journal and Los Angeles Times they had received no orders to arrest Khattalah or any other suspect in connection with the attack, which also killed three other Americans.
Khattalah may have fled the country, The New York Times said.
Possibly complicating matters, Ansar al-Sharia may have disbanded, Libyan state media reported.
The new disclosures highlight how problematic it is to investigate the Benghazi killings and bring the perpetrators to justice, the Journal said.
U.S. President Barack Obama has vowed several times -- including during Tuesday night's second presidential debate with Republican nominee Mitt Romney -- that his administration would fully investigate the killings and bring the perpetrators to justice.
"We are going to find out who did this, and we are going to hunt them down," Obama said.
Republicans have sought to tie the consulate attack to al-Qaida to counter Obama's assertion that by killing Osama bin Laden and other al-Qaida leaders his administration has crippled the militant Islamist group.
Ansar al-Sharia shares al-Qaida's Islamist militancy but is believed to be separate and independent from al-Qaida, The New York Times said. It also operates only in Libya, rather than globally, although groups with the same name are reputed to operate in Yemen and Tunisia.
Ansar al-Sharia means Partisans, or Supporters, of Islamic Law.
The FBI, which is conducting an investigation, declined to comment on Abu Khattalah or Ansar al-Sharia. The FBI is based in Tripoli, 400 miles away from Benghazi. It says Benghazi is too dangerous.
Ansar al-Sharia denied any involvement in the Benghazi violence.
Shortly after the Benghazi attack, U.S. spy agencies intercepted electronic communications of Ansar al-Sharia fighters bragging about their exploits to an al-Qaida in the Islamic Maghreb operative, The New York Times said Oct. 2.
Al-Qaida in the Islamic Maghreb is an al-Qaida affiliate originating in Algeria.
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