The investigation into what caused the crash of John F. Kennedy Jr.'s plane will be unusually complicated because the plane went down in the ocean, but experienced investigators from the National Transportation Safety Board can pull secrets from what appears to be a pile of junk.

The U.S. Coast Guard said yesterday that the wreckage had not yet been spotted, and with the water temperature at 68 degrees, it was unlikely that anyone could have survived more than 12 hours. The Kennedy and Bessette families were informed, and the search was officially transformed from a search-and-rescue mission to a search-and-recovery mission.

With the major portion of the wreckage still eluding the Coast Guard, evidence was disappearing hour by hour on the ocean floor, corroded by salt water, washed away by tides or even eaten by ocean creatures. It is already likely too late to collect some evidence from the bodies of victims, such as certain bodily fluids that could give investigators information not only about a person's condition but also whether there was fire or dangerous gases in the cockpit.

But while waiting for the plane and any bodies to be recovered, there is much investigators can do.

The area off Martha's Vineyard has an abundance of nearby radar sites that will allow investigators to reconstruct the last movements of the plane.

Already, the data have allowed the safety board to determine the course of the flight and its altitude. Investigators said yesterday the plane took off at 8:38 p.m. from Fairfield, N.J., flew north of nearby Teterboro Airport and then east along the southern coast of Connecticut at 5,600 feet.

Near Westerly, R.I., at 9:26 p.m., it began a descent toward Martha's Vineyard.

At 9:40 p.m., it was recorded at 2,500 feet. At 9:40 and 29 seconds, the next radar "hit" recorded it at 1,800 feet -- a descent rate of 1,400 feet per minute if it descended at an even rate. That is within the capabilities of the plane, but somewhat more steep than normal, according to pilots. A 600 feet-per-minute rate would be considered more comfortable for passengers.

Air traffic control voice tapes revealed that Kennedy made no radio calls in those final minutes. And people familiar with Kennedy's plane said it didn't have a voice recording device aboard.

The plane did have a Global Positioning Satellite navigation system. Sometimes called the "poor man's flight data recorder," a GPS system maintains a second-by-second recording of a plane's location, allowing a fairly accurate determination of flight path and speed -- if the GPS box can be found.

Assuming the plane is found and recovered, investigators will have a wealth of potential information in the cracks, nicks, tears and marks on the fuselage, interior parts and the engine.

They would first eliminate possible causes, keeping in mind that most crashes have several causes. They would try to determine whether the plane broke up in flight and whether a fire occurred either in the air or after the crash.

One of the first priorities for recovery is the propeller and engine. Hitting a body of water at high speed is almost like smashing into concrete, and a metal propeller such as the one on Kennedy's plane will bend in ways that can tell investigators whether it was turning or stopped.

The engine is the most rugged part of the airplane, and it likely survived the crash largely intact. Investigators can often spot an engine problem from an external inspection. In high-profile cases such as this one, the engine is often sent back to the manufacturer for a detailed breakdown.

Investigators can tell a lot by examining breaks and tears in the plane, sometimes by looking for paint smears or deep scratches on the surface that would be caused by parts that broke off the plane and hit other parts of the aircraft. Similarly, there are telltale marks for bending, torsion and shears, and unmistakable signs of fire. For instance, when heated, aluminum will merely drop into puddles in a ground fire. But in the air, the wind stream will stretch the hot aluminum into hundreds of small thread-like formations called "broomstrawing."

Plastic, wood, cloth and composite materials also break and tear in recognizable ways.

Cockpit instruments can yield "witness marks" if the hands of instrument dials are crushed into the displays.

Normally, a crash investigation also delves into a pilot's physical and mental health, concentrating on all of his activities from 24 to 72 hours before the crash.

Friday 9:26 p.m.: Near Westerly, R.I. Plane is at 5,600 feet and begins to descend.

Friday, 8:38 p.m.: Kennedy plane departs New Jersey.

Friday, 9:40 p.m.: Plane has descended to 2,500 feet 17 to 18 miles west of Martha's Vineyard airport. Twenty-nine seconds later, the plane has dropped to 1,800 feet.

Saturday afternoon: Debris, including luggage, plane parts and a prescription bottle, washes ashore on Philbin Beach.

Yesterday: Plane insulation washes ashore.