Secretary of State Hillary Clinton announced the agents' arrival in Libya Tuesday, but did not say how many agents were there or whether they had arrived in Benghazi.
"The FBI has joined the investigation on the ground in Libya and we will not rest until the people who orchestrated this attack are found and punished," Clinton told reporters.
U.S. Ambassador to Libya Christopher Stevens and three other State Department employees were killed last week when a protest linked to the U.S.-made video, "Innocence of Muslims," that debases the Prophet Muhammad turned violent and an armed mob rushed the consulate.
Some officials said they believe militants used the protests as a cover to attack and kill Stevens and the three others.
However, CNN reported White House spokesman Jay Carney saying nothing points to the attack being preplanned.
"We saw no evidence to back up claims by others that this was a preplanned or premeditated attack," Carney said. "But there is a lot that is under investigation here, and as more facts come to light, if they change that assessment, we'll make that clear."
Libyan officials said up to 50 people have been held for questioning. Most of them were considered as witnesses who were outside of the diplomatic compound during the protests, the officials said.
Arab-Israeli mayors and religious leaders, led by Knesset member Taleb a-Sanaa, sued U.S. search engine giant Google, demanding that it remove an anti-Islam video.
In their lawsuit, filed Wednesday in the Jerusalem District Court, the plaintiffs said the video posted on the Google subsidiary YouTube that denigrates the Prophet Muhammad violates at least three local laws, The Jerusalem Post reported.
The lawsuit claimed the publication of the video violates the religious feelings of Muslims, incites racism against Muslims and is defamatory.
News reports indicated Google blocked access to the video in Egypt, Libya, Indonesia and India, but rejected a White House request to pull it from YouTube altogether, the Post said.
Afghan officials ordered YouTube blocked "indefinitely," while Russia and Saudi Arabia threatened to block the website unless it removes the film.
In Cairo, Egypt's public prosecutor charged seven Coptic Christians living in the United States and a Florida pastor with insulting Islam and inciting sectarian strife for their alleged links to the video, CNN reported.
The move is largely symbolic since none of those charged lives in Egypt, observers said.
Among those charged were alleged filmmaker Nakoula Basseley Nakoula, identified by Egyptian officials as Elia Bassili; Morris Sadek, believed to be the one who posted the clip on YouTube, and Florida pastor Terry Jones, who allegedly was contacted by the filmmaker to help promote the video. CNN said.
Nakoula has denied he made the film.
The men also were charged with harming national unity and spreading false information, the prosecutor's office said.
Meanwhile, a man whose studio was used in the making of "Innocence of Muslims" said he was misled by the person behind the film, CNN said.
Joseph Nassralla, president of Media for Christ, based in Southern California, said he was forced into hiding because of death threats.
Nassralla, in a statement, said Nakoula told him he was making a movie about Christian persecution that "would examine the culture of the desert and how it is related to what is going on right now."
"Nakoula altered the film without anyone's knowledge, changing its entire focus and dubbing in new dialogue ...," Nassralla said. "The final product ... bore no resemblance to the film I thought he was making, or the film the actors thought they were creating. We were shocked."
An al-Qaida affiliate in North Africa Tuesday urged Muslims in the region to kill U.S. government representatives, CNN said.
"We encourage all Muslims to continue to demonstrate and escalate their protests ... and to kill their [American] ambassadors and representatives or to expel them to cleanse our land from their wickedness," the militant organization said in a statement.
In Tunisia, Foreign Affairs Minister Rafik Abdessalem said Tuesday his country will cover the damage done to the U.S. Embassy and the American Cooperative School in Tunis stemming from vandalism and arson at the facilities Friday, Tunisia Live reported.
Abdessalem met with U.S. Ambassador to Tunisia Jacob Walles Monday.
"We [the Tunisian government] expressed our regret and condemnation of what happened on Friday to Ambassador Walles," Abdessalem said during a news conference Tuesday. "We are committed to cover the physical damage caused by the protest."
Abdessalem also stressed that the Tunisian government was committed to protect U.S. citizens and facilities.
Abdessalem said he spoke with Clinton, who reassured him that the United States will support Tunisia's transition to democracy.