The U.S. military now believes that damage to the French Embassy and a residential neighborhood in Tripoli during Monday night's raid on Libya was caused by an Air Force "smart" bomb that went astray either because it was dropped by a damaged F111 jet or because its guiding laser beam was blocked by clouds, Defense Department officials said yesterday.

Preliminary assessments also show that Air Force and Navy planes substantially damaged four of their five military targets, Pentagon officials said. Clouds or smoke obscured a fifth site, a training base at a port near Tripoli, and several Air Force pilots -- under strict orders not to strike without a clear view of their targets -- had to fly away without releasing their bombs, officials said.

The loss of one F111 with a two-man crew and the bombing of the residential neighborhood, which Libya says killed and injured dozens of civilians, were the only major flaws in a complex nighttime operation involving more than 100 planes, according to the Defense Department. The search for the missing bomber was abandoned yesterday.

For the most part, officials said, the operation went smoothly, with Navy and Air Force planes cooperating well and using smarter tactics than were employed in a 1983 raid on Lebanon.

"High technology pays off handsomely indeed, as it will again if it has to be used," Defense Secretary Caspar W. Weinberger said in a Cambridge, Mass., speech last night.

"There's been a wholesale change in tactics in a very short time," Navy Secretary John F. Lehman Jr. said in an interview.

Weinberger said shortly after the raid Monday night that it would have been "virtually impossible" for U.S. bombs to have damaged the French Embassy. Pilots aboard the aircraft carrier USS America suggested Tuesday that damage to neighborhoods was caused by Libyan antiaircraft missiles falling back to earth.

Pentagon officials agreed yesterday that misguided Libyan missiles probably caused some damage. But they also said that the explosion near the French Embassy was likely caused by a 2,000-pound Air Force bomb.

One explanation focused on the missing F111 fighter, which probably crashed into the Mediterranean Sea off Tripoli, officials said.

The Defense Department originally thought the plane was lost before it reached its target, but officials yesterday said they believe it was hit over Tripoli and then turned north before it exploded, perhaps as the pilot tried to reach water before ejecting.

Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, said yesterday that there is a "very definite" possibility that the F111 was hit by antiaircraft fire, although he said he is not certain. Crowe said that he is "confident that the crew of the plane was lost."

"We do have some pilots that were witnesses to a fireball going into the water off Tripoli," Crowe said on the NBC "Today" program. "We believe that was the aircraft, but, again, the debrief of the pilots is not complete as yet."

Other officials said that if the F111, flying low over Tripoli, was hit as it released a bomb, the ordnance might have gone astray.

A second explanation is also consistent with the likely trajectory of the bomb, however. The 2,000-pound GBU10 bombs are designed to home in on a beam of light with which the "Pave Tack" system on the plane's underbelly focuses on the target.

After the bomb was dropped, the F111 probably swerved and climbed to evade antiaircraft fire, while the laser designator on its undercarriage automatically swiveled to keep the target illuminated.

As the plane moved, however, the laser beam may have been broken by smoke or clouds that were drifting over Tripoli Monday night, causing the bomb to fall unguided into the residential neighborhood.

Clouds or smoke also interfered with the F111 raid over the Sidi Bilal port, where the target was a commando training post. As the F111s roared in at treetop level -- flying at 9 miles per minute, Weinberger said -- broken clouds or smoke from other explosions prevented their lasers from "locking on" the targets, officials said.

As a result, the training camp was not damaged and some F111 pilots -- after flying six hours from Britain -- had to fly off, probably to jettison their bombs in the ocean before the eight-hour return trip. But one alternate target at Sidi Bilal, the Libyan naval academy, was heavily damaged.

"That wasn't the intended target, but they did a good job of it," one officer said.

The Pentagon declined to officially release any bomb damage assessment yesterday, but the Joint Chiefs of Staff began to evaluate the mission's success based on reconnaissance photographs, infrared film shot from the bombers and other intelligence.

Gen. Charles A. Gabriel, Air Force chief of staff, personally brought some of the infrared footage back from Britain, where he had been on unrelated business.

The bomber cameras, which operate at night by sensing the heat of the bombs and landscape, recorded several bombs falling into the compound where Libyan leader Muammar Qaddafi's family resides.

Cloud cover at the Tripoli airport, which F111s also bombed, prevented reconnaissance photography. But other intelligence sources indicated that the raid damaged at least six of the 11 IL76 Soviet-built cargo planes at the airport.

Navy A6 Intruders, which attacked two sites in eastern Libya while the F111s bombed near Tripoli, caused considerable damage, according to post-attack photographs.

The Jamahiriyah Barracks were "pretty well pasted," one official said, and at the Benina airport -- where the Intruders dropped 750-pound cluster bombs -- the runway was somewhat cratered and four helicopters and 13 airplanes, including some MiG23s and two commercial craft, were damaged.

In addition, Navy planes firing antiradar missiles destroyed at least five antennas at Libyan surface-to-air missile (SAM) sites and probably succeeded in shutting several others down, officials said. F/A18 Hornets, shooting from as far as 40 miles, fired High-speed Anti-Radiation Missiles (HARMs), while A7 Corsairs fired older Shrike missiles from about 8 miles away.

The antiradar missiles do not destroy the SAM sites, but -- combined with electronic jamming and spoofing from EA6B Prowlers and other planes -- made targeting virtually impossible for the Libyans. As a result, officials said, Libyan missiles were simply launched in a barrage that had less chance of success.

Lehman said the tactics and targets took advantage of U.S. strengths, particularly in low-altitude nighttime flight, which demands sophisticated ground avoidance radar, and in electronic warfare. In Lebanon, by comparison, Navy planes attacked by day, using Vietnam-era tactics, he said.

In addition, he said, the Navy and Air Force had no difficulty communicating during the raid, as carrier-based fighters flew watchfully over the bombers and Navy planes helped suppress defenses where Air Force planes were attacking. The services have been criticized before for not working well together.

Altogether, 24 F111s, including six "airborne spares," took off from Britain, supported by dozens of tanker planes for in-flight refueling. The spares turned back at the first refueling station and two other planes had to turn back subsequently due to mechanical problems, leaving 16 to attack Libya.

The Navy attacked with 14 A6s.