Benghazi, Libya (CNN) -- FBI investigators probing the U.S. Consulate killings in Libya put off a visit there until conditions in the volatile region are safer.
Agents hoped to arrive on Saturday but reconsidered because of the instability sweeping across Libya and throughout the region, federal law enforcement officials said.
One source said there's nothing to be gained by putting additional people in harm's way when the situation on the ground remains volatile. It's hard to say when the situation will become stable enough for the FBI to visit, the officials said.
"Safety comes first," one source said.
Agents are now conducting interviews with witnesses outside the country, including people who've been evacuated.
Libyan officials believe it was a planned assault that used a protest over a film mocking the Prophet Mohammed as a diversion. Rage over the film across the Muslim world has made the region dangerous terrain for Westerners.
The federal sources say the crime scene at the consulate is far from pristine. They say many people, including journalists, have been inside the charred remains of the compound, but they insist evidence can still be gathered.
Don Borelli, a retired FBI agent, said the bureau will want to keep a low footprint but create a team big enough to get the job done. Agents must consider security and logistical issues, he said.
"The more people you bring over there, the more people you have to worry about," he told CNN.
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.
Four Libyans have been 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.
Arwa Damon reported from Benghazi and Susan Candiotti reported from New York. CNN's Chelsea J. Carter, Jomana Karadsheh, Elise Labott, Brian Walker and Ross Levitt also contributed to this report.