(CNN) -- There is a connection between two Tunisians detained for questioning in Turkey and the attack on an American Consulate in Libya that left four dead, the U.S. defense chief said.
"We know there is some connection but, frankly, we really don't have all the specifics," Secretary of Defense Leon Panetta told reporters Friday.
The Tunisians -- who had been on a watch list provided by the United States to Turkish authorities -- are being questioned at the request of the United States, said a U.S. official with knowledge of the investigation. They entered Turkey this week.
FBI investigators have not talked to the two Tunisians yet, but "that's the hope," said the U.S. official, who spoke on condition of anonymity. The official was not authorized to release the information.
The official did not confirm or deny if the suspects entered Turkey using fake passports, as has been reported by Turkish media.
The September 11 consulate attack killed U.S. Ambassador to Libya Chris Stevens and three other Americans. The incident heightened global scrutiny of the North African nation and sparked debate about whether members of President Barack Obama's administration were being forthcoming about the incident.
In the days after the assault, U.S. administration officials offered conflicting assessments on what may have led to the fatal security breach.
Officials initially said the violence erupted spontaneously amid a large protest about a privately made video produced in the United States that mocked the Prophet Mohammed.
But the U.S. intelligence community revised its assessment, saying it believes it was a planned terrorist assault.
The intelligence community now believes it was "a deliberate and organized terrorist assault carried out by extremists" affiliated with or sympathetic to al Qaeda.
For the first time, an FBI team spent "a number of hours" this week at the Benghazi attack site, Pentagon spokesman George Little said. They were accompanied by what Little described as a "small footprint of (U.S.) military personnel."
While FBI agents have been absent, U.S. Special Operations forces have been in Libya, as well as nearby countries, to help collect intelligence about the assault, a U.S. military official told CNN on Thursday. The official declined to be identified due to the sensitive nature of the information.
Officials said the military presence was an indication of ongoing security concerns in the region, which is a major reason why it took FBI agents three weeks to visit the attack site. That gap, however, has raised questions about the integrity of the FBI investigation and concerns that sensitive documents may have been left unsecured.
Three days after the attack, CNN Senior International Correspondent Arwa Damon discovered Stevens' journal during a visit to the unguarded, abandoned compound.
This week, a Washington Post reporter visiting the site found sensitive documents, including personnel records of Libyans who had been contracted to provide security, emergency evacuation protocols, and details of U.S. weapons collection efforts.
But a State Department official told CNN that no classified documents had been left on the premises.
Questions have been raised, meanwhile, about whether there was adequate security for U.S. diplomats and missions before Stevens and the three others were killed.
An internal State Department e-mail -- provided to CNN by a U.S. government source -- shows the State Department earlier this year denied a request by the security team at the U.S. Embassy in Libya for an airplane to transport security personnel and for diplomatic business. Stevens was copied on the e-mail, which was signed by Miki Rankin of the State Department's Near East Bureau.
State Department spokesman Mark Toner told reporters Friday that the decision not to keep a DC-3 plane in Tripoli -- and use charter flights, instead, if needed -- "is a very common practice" in places where commercial airline service is available.
U.S. Special Forces troops went to the U.S. Consulate in Benghazi soon after an April attack on a U.N. convoy spurred concerns about security there, an Obama administration official said. The U.S. military team went there to assess the situation and train local Libyan forces on how to better protect the facility.