BENGHAZI, Libya (CBS/AP) -- U.S. officials increasingly believe that the attackers who stormed a U.S. Consulate in Libya and killed an American ambassador may have had terrorist links and were not solely members of a spontaneous mob demonstrating against an online video ridiculing Islam's Prophet Muhammad.
A U.S. source told CBS News investigative producer Pat Milton that assessments are still under way on how the attack unfolded. The source said it is appearing more and more like the attack was one of opportunity rather than pre-planned, but nothing has been determined yet.
U.S. Ambassador to Libya Christopher Stevens and three other Americans died in the attack on the U.S. Consulate in the eastern Libyan city of Benghazi Tuesday.
House Intelligence Committee Chairman Mike Rogers, R-Mich., told CBS News Capitol Hill producer Jill Jackson there was a "high likelihood" that the attack had an affiliation with "al Qaeda elements in Libya."
"This was a coordinated attack, more of a commando-style event," Rogers told Jackson. "It had both coordinated fire -- direct fire and indirect fire. There appeared to be military maneuvers approaching the facility."
CBS News national security correspondent David Martin reports the FBI has opened an investigation into the killings and will send agents to Libya to search for evidence. At the same time, the U.S. will increase its surveillance over Libya, including the use of unmanned drones the Central Intelligence Agency has used against terrorists in Pakistan. Finally, the U.S. Navy is positioning two destroyers armed with cruise missiles off the coast of Libya.
Wanis al-Sharef, a Libyan Interior Ministry official in Benghazi, said there had been threats that Islamic militants might try to take revenge for the death of al Qaeda's No. 2 commander Abu Yahya al-Libi, who was killed in a U.S. drone strike in Pakistan in June, and he said the U.S. consulate should have been better protected.
Confirming al-Libi's death for the first time in a video posted online Monday, al Qaeda chief Ayman Al-Zawahri called on Muslims in al-Libi's native Libya to take revenge for his death.
U.S. officials believe the militants were using the demonstration against the video as a cover to get into the consulate and then take as much revenge as they could on Americans, Martin reports.
"Absolutely it's a terrorist attack," Rogers told Jackson. "This was not done by the Libyan government. It was done by an external group we believe has at least extremist ties, maybe al Qaeda ties, and the style and the signature of the attack clearly would be something that we have seen before and would be in line with something al Qaeda would do."
Military officials told CBS News an anti-terrorism team of U.S. Marines was being deployed to Libya to help secure U.S. interests in the country following the attack. The State Department said, however, that no Americans were remaining at the facility in Benghazi. State officials would not confirm how many Americans were evacuated, or to where.
Al-Sharef said Stevens, 52, and other officials were moved to a second building - deemed safer - at the consulate compound after the initial wave of the attack. According to al-Sharef, members of the Libyan security team seem to have indicated to the attackers the building to which the American officials had been relocated, and that building then came under attack.
Stevens was the first U.S. ambassador to be killed in the line of duty since 1979. A Libyan doctor who said he treated Stevens told The Associated Press Wednesday that the diplomat died of severe asphyxiation and that he tried for 90 minutes to revive him.
Ziad Abu Zeid said Stevens was brought to the Benghazi Medical Center by Libyans Tuesday night with no other Americans, and that initially no one realized he was the ambassador. Abu Zeid said Stevens had "severe asphyxia," apparently from smoke inhalation, causing stomach bleeding, but had no other injuries.
Al-Sharef said two U.S. Marines sent to Benghazi when the clash erupted were shot and killed. It was not immediately clear whether the Marines were part of Stevens' security detail. The American whose death was confirmed on Tuesday also died of a gunshot wound. He was identified by the State Department on Wednesday as Foreign Service Information Management Officer Sean Smith.
CBS News confirmed that the two other Americans who died in the attack were Marines. All four bodies have been taken to Tripoli.
Deputy Libyan Prime Minister Mustafa Abushagur said on his Twitter account that, "Amb. Stevens was a friend of Libya and we are shocked at the attacks on the U.S. consulate." The Libyan Prime Minister also expressed grief and condemned the attack.
According to his biography page on the U.S. Embassy's website, Stevens "was the American representative to the Transitional National Council in Benghazi during the revolution," in Libya. Benghazi was the capital of rebel-held Libya during the uprising to oust Libyan strongman Muammar Qaddafi.
According to al-Sharef, the attackers stormed the consulate after the U.S. troops who responded fired rounds into the air to try and disperse the crowd. Al-Sharef said the Libyan guards employed to guard the consulate building were far outgunned by the protesters, and thus retreated when the building was stormed.
Hours before the protest erupted in Benghazi, protesters scaled the walls of the U.S. Embassy in Cairo, Egypt, tearing down and replacing the American flag with an Islamic banner.
CBS News' Alex Ortiz reported from Cairo that about 40 or 50 protesters had again gathered in front of the U.S. Embassy on Wednesday, but they were far outnumbered by Egyptian security personnel.
Tuesday's attacks were the first such assaults on U.S. diplomatic facilities in either country, at a time when both Libya and Egypt are struggling to overcome the turmoil following the ouster of their longtime leaders, Qaddafi and Hosni Mubarak, in uprisings last year.