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(CNN) -- U.S. Secretary of State Hillary Clinton said Thursday she has "absolutely no information or reason to believe there is any basis" to suggest that U.S. Ambassador to Libya Chris Stevens believed he was on an al Qaeda hit list.
The remark came after a source familiar with Stevens' thinking told CNN that in the months leading up to his death, Stevens worried about constant security threats in Benghazi and mentioned that his name was on an al Qaeda hit list.
Stevens spoke about a rise in Islamic extremism and al Qaeda's growing presence in Libya, the source said.
The White House, for the first time Thursday, declared the attack that killed Stevens and three other people a terrorist attack.
"It is, I think, self-evident that what happened in Benghazi was a terrorist attack," said White House spokesman Jay Carney, speaking to reporters on Air Force One.
On Wednesday, Matthew Olsen, director of the U.S. National Counterterrorism Center, gave a similar statement.
"I would say yes, they were killed in the course of a terrorist attack on our embassy," Olsen said at a Senate Homeland Security Committee hearing.
Stevens and three other Americans were killed September 11 during a large protest at the U.S. Consulate in Benghazi. Demonstrators were angry about a film made in the United States that mocked the Muslim Prophet Mohammed.
President Barack Obama, during a candidates forum Thursday on Univision, said the anti-Islam video, "Innocence of Muslims," that stirred protests was "used as an excuse by some to carry out inexcusable violent acts directed at Westerners."
Sources tracking militant Islamist groups in the region said that a pro-al Qaeda group was the chief suspect and that the attack appeared to have been planned. The attackers used the protest as a diversion, the sources said.
At a memorial service Thursday in Libya, Mohamed al-Magariaf, president of Libya's General National Congress, expressed sorrow for the deaths of the four who were working "for the sake of Libya."
Libyan and U.S. officials attended the service in Tripoli for Stevens, computer expert Sean Smith, and former Navy SEALs Glen Doherty and Tyrone Woods.
Stevens "helped Libya in its darkest hours," al-Magariaf said.
"He worked tirelessly as a representative of the United States of America."
The U.S. national anthem played at the ceremony.
Al-Magariaf vowed the perpetrators will be brought to justice.
Libya has taken steps to arrest those responsible for the consulate attack, bringing in dozens for questioning over the weekend, Libyan officials have said.
Prime Minister Mustafa Abushagur told CNN on Thursday that eight people have been detained in the investigation.
Those in custody include "far extremists" including members of Ansar al-Sharia, a loosely connected radical Islamist group based in Benghazi, he said. Yet while some Ansar al-Sharia members may have been among the 30 to 50 people involved in the September 11 assault, the attackers didn't all come from one specific group, Abushagur added.
The prime minister described those behind the attack as a loose organization with no set members, no known affiliation to al Qaeda and no foreigners among their membership. A Libyan official earlier said those arrested included some from Mali and Algeria, as well as al Qaeda sympathizers.
A senior U.S. official, meanwhile, told CNN on Thursday that Ansar al-Shariah had not been positively identified as responsible for the attack, "which is more likely to turn out to be a bunch of various elements and basically (al Qaeda) militants."
Authorities are going after those who led the attack, though Abushagur said Libya is a vast country with many places to hide.
As to taking on extremist militias, generally, around Libya, the prime minister said his government's goal is to bring them into the mainstream, and that taking them on militarily would lead to unnecessary bloodshed.
Abushagur praised the Obama administration's handling of the situation. Still, while he welcomed general cooperation, the prime minister said that U.S. military action in Libya would spark chaos.
"We are against activity by any foreign country, because this is, of course, our sovereignty," the prime minister said. "We are prepared to handle this situation."
CNN's Arwa Damon contributed to this report.