An unmanned recovery vehicle pinpointed the location of EgyptAir Flight 990's vital onboard recorders today, but the Navy reported that they are buried under silt and perhaps some aircraft wreckage.

The Deep Drone, owned and operated by Navy contractor Oceaneering Engineering of Upper Marlboro, Md., scratched around in the silt with its movable arm in an effort to find the cockpit voice recorder and flight data recorder but stopped work about 7 p.m. (EST) because the weather turned bad.

Navy Capt. Bert Marsh said the Deep Drone's operators are confident they are close--perhaps within 15 feet--to the recorders, or "black boxes," as the orange-painted devices are called. He said the Deep Drone was getting a lusty signal from the boxes' sonar locator "pingers."

"The level of the sound was such that they had to take their earphones off," Marsh said.

But he added, "They have had no joy in finding either of the pingers [boxes] just yet. . . . But they know they're there."

The wide-body Boeing 767 went down early Sunday with 217 people on board, inexplicably diving steeply from 33,000 feet on a flight from New York to Cairo. All passengers and crew died.

National Transportation Safety Board Chairman Jim Hall expressed confidence in an Air Force radar study that indicated the plane dived rapidly from 33,000 feet to about 16,000 feet, then climbed back up to 24,000 feet before diving again to 10,000 feet--where it either broke up or began shedding significant pieces of its airframe.

Many outside radar and aircraft experts have expressed skepticism that the plane could have maneuvered that way without breaking up long before it apparently did. They were also skeptical that the available radar data was accurate enough to draw those conclusions so early in an investigation.

However, Hall said the Air Force's 84th Radar Evaluation Group had rechecked its data several times and was confident it was correct. Also, he said safety board specialists had performed a computerized simulation that indicated the plane was capable of flying those maneuvers.

"It appears from those simulations that a 767 can fly that altitude profile," Hall said.

However, only the plane's onboard recorders--if they have good data--can tell investigators exactly how the plane maneuvered and why it began its abrupt dive.

So far investigators are flummoxed, although speculation on a cause has become almost a sport in the aviation community. The speculation ranges from an onboard shooting incident to some sudden first-time mechanical problem, or perhaps a problem that led the pilots to descend so rapidly that they lost control.

An Atlantic gale has hindered the search for the recorders, which are the board's highest priority. But the Navy salvage ship USS Grapple was able to put to sea Thursday night with the Deep Drone, and unexpectedly good weather allowed the search to continue much longer than expected.

The investigation also received a major boost when the Deep Drone discovered that the sea bottom was not cluttered with tangles of wire and wreckage that proved to be a complication in the recovery of Trans World Airlines Flight 800 in 1996.

Marsh said the sea bottom in the vicinity of the buried boxes contained "minor pieces of debris, not in a large section." The largest piece was an aircraft wheel.

However, he would not speculate whether the recorders might be entangled with other buried wreckage.