A top explosives and chemical weapons expert in Al Qaeda and a relative of the terror network's No. 2 leader, Ayman al-Zawahiri, were among the top operatives believed killed in a U.S. missile strike in Pakistan last week, Pakistani security officials said Thursday.

The three security officials, all speaking on condition of anonymity because they were not authorized to speak to media, named and described three of the terrorists they believe were killed. Some Pakistani officials say it's also believed that Mustafa Usman, an Egyptian with possible links to Zawahiri, Usama bin Laden's top lieutenant, was also killed.

Other officials also said a fifth man, Khalid Habib, the Al Qaeda operations chief along the Afghan-Pakistan border, is believed to be dead.

While officials are "pretty sure" that three of the men are dead, they are less certain about the fate of Usman and Habib, FOX News has learned.

None of the deaths could yet be confirmed by Pentagon; CIA spokesman Tom Crispell said the agency could not comment.

A spokesman for Pakistan's President Gen. Pervez Musharraf, Maj. Gen. Shawkat Sultan, told FOX News Thursday morning he could not identify those killed but that the attack is "under investigation."

The Al Qaeda figures were believed to have been in Damadola village near the Afghan border at the time of Friday's attack but their bodies have not been recovered. The attack also killed 18 civilians.

Further clouding exactly what happened, according to FOX News sources, is that there was a three- to four-minute gap between the first and second missile strike. It is thought that some individuals in a targeted house might have been able to escape following the first strike.

The officials said the operatives included Midhat Mursi al-Sayid Umar, 52, who the U.S. Justice Department calls an explosives and poisons expert. The Egyptian also has distributed training manuals with recipes for chemical and biological weapons and trained hundreds of fighters at a terrorist camp near the eastern Afghan city of Jalalabad before the ouster of the hardline Taliban regime in late 2001.

Umar is suspected of training the suicide bombers who killed 17 U.S. sailors in the attack on the USS Cole in Yemen in 2000, according to Mohamed Salah, a Cairo expert on Islamic extremists.

The Justice Department's Web site says the exact whereabouts of Umar, also known as Abu Khabab al-Masri, are unknown but that he may be living in Pakistan. It offers $5 million for information leading to his arrest.

Pakistani officials said the other militants possibly killed included Abu Obaidah al-Masri, the Al Qaeda chief responsible for attacks on U.S. forces in eastern Afghanistan, across the border from the strike site; and Abdul Rehman al-Maghribi, a Moroccan who is believed to be al-Zawahiri's son-in-law.

One of the officials said al-Maghribi was involved in public relations for Al Qaeda and helped distribute statements, CDs and videos publicizing the group. In particular, al-Maghribi had contacts with Arab journalists and kept them abreast of Al Qaeda news, he said.

The officials said Habib had planned assassination attacks on Musharraf and is associated with Abu Farraj al-Libbi, a top Al Qaeda figure arrested in northwestern Pakistan in May.

FOX News has learned that Pakistan intelligence agencies have been "liasing" with U.S. authorities and that the names of at least some of the victims had been "floating around" prior to the attack. At least two of them were expected to be at the gathering that was targeted. More definitive word based on new intelligence and evidence being gathered should be emerging in the next "day or two," according to the FOX News source.

Pakistani authorities previously said four or five foreign militants were killed in the airstrike, which officials say targeted — but missed — al-Zawahiri. The strike has angered many in the Islamic country, prompting street protests over the weekend.

About 1,000 protesters also marched through the northwestern city of Peshawar on Thursday, chanting "Death to America" and "Jihad, Jihad."

Pakistan maintains it was not given advance word of the airstrike, which was reportedly carried out by unmanned Predator drones flying from Afghanistan, and has condemned the killing of innocent civilians.

Provincial authorities said Al Qaeda sympathizers took the bodies of the foreign militants believed to have been killed to bury them in the mountains near the Afghan border, thereby preventing their identification.

Pakistani Interior Minister Aftab Sherpao told The Associated Press on Wednesday that the bodies may have been taken by a local pro-Taliban cleric, Maulana Faqir Mohammed, who also is being hunted by authorities. Authorities believe he and another prominent pro-Taliban cleric survived the attack Friday.

While a FOX News contact has received multiple confirmations that bodies were removed from the site before investigators could get to them, investigators do have some sort of forensic evidence, perhaps remains of bodies at the sites, and could be used, through DNA testing, to confirm the identities of those killed.

Intelligence officials say al-Zawahiri is thought to have sent some of his aides in his place to an Islamic holiday dinner to which he'd been invited in Damadola on the night of the attack.

Hours after the attack, an Associated Press reporter visited the village, which consists of a half-dozen widely scattered houses on a hillside about four miles from the Afghan border.

Residents said then that all the dead were local people and no one had taken any bodies away. However, it appeared feasible that bodies or wounded could have been taken away in the darkness after the attack, which took place at about 3 a.m.

Islamic custom dictates that bodies be buried as soon as possible, and the reporter saw 13 freshly filled graves with simple headstones and five empty graves alongside them — apparently prepared for more dead. When the reporter returned the next day, the five empty graves were filled in, apparently because no more bodies had been found in the rubble.

FOX News' Greg Palkot and The Associated Press contributed to this report.