A desk inside the burnt U.S. Consulate building in Benghazi, Libya, on Thursday, September 13, two days after an attack on the building in which the U.S. ambassador and three other U.S. nationals were killed. The attack came as protesters outside the compound rallied against a movie that unflatteringly portrays Islam's Prophet Mohammed. Photos: Protesters storm U.S. Embassy buildings 
The damage inside the burnt U.S. Consulate in Benghazi on Thursday. 
A lounge chair and umbrella float in the swimming pool of the U.S. Consulate in Benghazi on Thursday. 
Demonstrators on Wednesday, September 12, gather in Libya to condemn the killers and voice support for the victims in the attack on the U.S. Consulate. 
U.S. President Barack Obama makes a statement about the death of Ambassador Chris Stevens with Secretary of State Hillary Clinton in the Rose Garden at the White House on Wednesday in Washington. 
A burnt vehicle is seen at the U.S. Consulate in Benghazi, Libya, on Wednesday. 
People inspect the damage at the U.S. Consulate in Benghazi, Libya, on Wednesday. 
A small American flag is seen in the rubble at the U.S. Consulate on Wednesday. 
President Barack Obama and Secretary of State Hillary Clinton stand at Andrews Air Force Base as the bodies of the four Americans killed at the U.S. Consulate in Benghazi are returned. 
A man stands in part of the burned-out compound Wednesday. 
Smoke and fire damage is evident in this consulate building. 
Half-burnt debris and ash cover the floor of one of the consulate buildings. 
The U.S. Consulate in Benghazi is seen in flames Tuesday, September 11. 
A protester reacts as the U.S. Consulate in Benghazi burns Tuesday night. 
A vehicle and the surrounding area are engulfed in flames after it was set on fire inside the compound Tuesday. 
Flames erupt outside of a building in the U.S. consulate compound on Tuesday. 
A vehicle burns during the attack Tuesday on the U.S. Consulate in Benghazi. 
Onlookers record the damage from the attack on Tuesday. 
Onlookers walk past a burning truck and building in the compound on Tuesday. 
A vehicle sits smoldering in flames on Tuesday. 
People duck flames outside a consulate building on Tuesday. Photos: Protesters storm U.S. Embassy buildings 
- NEW: Yemen's al Qaeda affiliate suggests a "revenge" attack
- Libya is cooperating with the "U.S. side" in Tripoli and Benghazi, an official says
- Four people have been arrested in connection with the attack
- An announcement about arrests "is expected soon," a Libyan official says
Benghazi, Libya (CNN) -- The FBI was expected to arrive Saturday in Libya to investigate the attack that killed U.S. Ambassador J. Christopher Stevens and three other Americans, a U.S. official said.
The American investigators were believed to be headed to Benghazi where the U.S. consulate was attacked by what Libyan officials believe was a pre-planned assault that used a protest over a film mocking the Prophet Mohammed as a diversion.
A team of investigators was due to arrive in Libya after first conducting interviews outside the country, said the official with knowledge of the investigation. The official, who spoke on condition of anonymity, was not authorized to release details to the media.
Libya is cooperating "with the U.S. side now, both here and in Benghazi," Monem Elyaseer, the aide to the head of Libya's ruling General National Congress, told CNN on Saturday.
"Things are moving very, very well," Muhammad Alkari, spokesman for the prime minister's office, told CNN.
News of the investigation came a day after the remains of Stevens and the three others were returned to the United States, where President Barack Obama and Secretary of State Hillary Clinton were present for the transfer of the flag-draped caskets from an airport hangar to four hearses.
Funeral arrangements for the four men were pending Saturday.
The anticipated arrival of the FBI team follows word that four Libyans were arrested in connection with Tuesday's assault on the consulate, though they were not said to be directly tied to the attack.
The identities of the four suspects have not been released, though Elyaseer said "there is an active interrogation of suspects at this time."
"You can expect something to be announced soon about arrests," he said.
The Libyan government now believes the suspects are part of one of the many armed extremist groups operating especially in the eastern part of the country and Benghazi itself, Mohammed Al-Megaryef, the head of Libya's ruling General National Congress, told CNN on Friday.
Authorities believe the attack was planned and deliberately carried out to inflict maximum damage on key Western interests, particularly the United States, he said.
U.S. officials, however, have said they have no firm evidence that the attack was premeditated.
The Libyan government believes the attack was intended to drive a wedge between Americans and Libyans.
"We are doing our best to avoid further attacks," al-Megaryef said.
But he acknowledged that authorities had little capacity to defend against the powerful extremist groups.
Given what is known about al Qaeda in Libya, U.S. intelligence officials believe it is unlikely that an al Qaeda-affiliated group was behind the attack, a U.S. intelligence official told CNN on condition of anonymity. The official was not authorized to release the information.
The Yemen-based Al Qaeda in the Arabian Peninsula issued a statement Friday suggesting that the recent killing of militant Abu Yahya al-Libi could have prompted people "to take revenge upon those who make light of our religion and attack our prophet."
Al-Libi was the No. 2 man of the so-called core al Qaeda along the Afghan and Pakistan border. A CIA drone strike in Pakistan killed him in June.
Al Qaeda in the Arabian Peninsula -- which did not claim involvement in the Libya attack -- is known as a substantial threat both in Yemen and beyond its borders.
The United States deployed 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. warplanes participated in the NATO effort that 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.
CNN's Chelsea J. Carter, Susan Candiotti, Arwa Damon, Jomana Karadsheh, Elise Labott, and Brian Walker contributed to this report.
