A team of FBI investigators is expected to be in Libya by Saturday, a source with knowledge of the investigation told CNN. Agents are first conducting interviews outside the country to gather information about the attack, the source said.
"I want people around the world to hear me. To those that would do us harm -- no act of terror will go unpunished, it will not dim the light of the values that we proudly present to the rest of the world. No act of violence shakes the resolve of the United States of America."
Pentagon spokesman Little said U.S. authorities continue to investigate the attack and partner with other governments to determine what happened.
During an interview on CNNI's "Amanpour," Abushagur said Thursday that there had been one arrest early Thursday in Benghazi and that three or four others were being pursued.
"The evidence itself is based on mostly pictures that were taken around the compound at that time, and also through some witnesses," the prime minister said.
Conflicting theories flew in the hours after Stevens, another diplomat and two State Department security officers were killed late Tuesday in the eastern city of Benghazi.
They died amid a protest outside the U.S. Consulate over a film that ridiculed Muslims and depicted the Prophet Mohammed as a child molester, womanizer and ruthless killer.
"I think the degree to which we're able to update this information or deepen it, it's going to be in the context of beginning to interview our employees who are coming out and beginning to participate in the investigation that the Libyans are doing," U.S. State Department spokeswoman Victoria Nuland told reporters Thursday.
The demonstration was one of several protests across the region that day.
Protest as diversion
U.S. officials believe the attackers used the protest as a diversion.
Given what officials know about al Qaeda in Libya, intelligence officials believe it is very unlikely that core al Qaeda was behind the attack, a U.S. intelligence official told CNN on condition of anonymity. The official was not authorized to release the information.
Meanwhile, Shawn Turner, director of communications for U.S. National Intelligence, denied news reports that American officials had been warned of a possible attack.
"This is absolutely wrong," he said. "We are not aware of any actionable intelligence indicating that an attack on the U.S. post in Benghazi was planned or imminent."
The United States is deploying warships and surveillance drones in its hunt for the killers of the diplomatic staffers, and a contingent of 50 Marines has arrived to boost the security of Americans in the country.
The United States and Libya have embarked on a new relationship since rebels toppled longtime dictator Moammar Gadhafi last year.
U.S. and NATO warplanes helped the Benghazi-based rebellion against Gadhafi, who was wanted by the International Criminal Court for charges of crimes against humanity before he was killed in October.
The jihadists suspected in Tuesday night's attack "are a very small minority" who are taking advantage of a fledgling democracy, said Ali Suleiman Aujali, the Libyan ambassador to the United States.