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.
Libyan 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.
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 pre-meditated.
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 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.