orty-eight hours after John F. Kennedy Jr.'s plane plunged into the ocean off Martha's Vineyard, federal officials last night ended their search for survivors, effectively declaring Kennedy, his wife, and her sister the victims of a fatal crash.
The move came in the form of an announcement by Coast Guard officials that the intensive, multiagency mission had shifted from ''search and rescue'' to ''search and recovery,'' a bureaucratic way of saying hope had ended of finding them alive.
''This is not the result that we were looking for. This is not the result we were hoping for,'' said Coast Guard Rear Admiral Richard Larrabee. ''We did everything we possibly could to find survivors from this incident.''
Meanwhile, Larrabee said divers from the Massachusetts State Police would begin work today investigating two ''potential targets'' identified by sophisticated sonar as unusual objects on the ocean floor.
He cautioned that, based on the information available last night, the targets ''don't in any way represent the location of an aircraft or parts of it.'' He said the objects were in 60 to 80 feet of water.
Larrabee said the decision to end the rescue efforts was made in consultation with the National Transportation Safety Board and had been communicated to Kennedy's family and the family of his passengers: his wife, Carolyn Bessette Kennedy, and her sister, Lauren Bessette.
''I've spent some very painful moments with the families tonight,'' a somber Larrabee said. ''It was very difficult for me to share the information with them.''
He said the move was based on four factors: the expected length of survival time in the 68-degree waters is only 12 to 18 hours; the plane had no emergency survival equipment; the two days of searching found nothing to indicate anyone had survived; and the NTSB's experience in assessing the probability of survivors in this type of crash.
Meanwhile, federal safety investigators said a voice recorder aboard Kennedy's small plane might provide clues to the Friday night crash.
Also yesterday, questions arose whether search and rescue operations might have begun five hours earlier had proper procedures been followed when a Martha's Vineyard Airport employee tried to alert authorities that the flight was overdue.
Even before Larrabee's announcement, the mood grew increasingly somber at the Kennedy family's Hyannis Port compound, where three priests clad in white celebrated Mass under a white tent. Senator Edward M. Kennedy held a wine chalice while one of the priests distributed Communion wafers. Ethel Kennedy, the widow of Robert F. Kennedy, also served consecrated wine to the dozen or so people who attended the service.
At an afternoon press briefing, the chairman of the NTSB cautioned that the investigation into the crash would take six to nine months, and the cause might remain elusive. ''At this point we do not know. We will not know for some time. There is even a possibility we will never know,'' NTSB Chairman James Hall said.
Yesterday, Coast Guard searchers turned up only bits of a headrest and foam insulation they believe came from Kennedy's single-engine airplane. The insulation, in pieces ranging from 2 to 10 inches in diameter, was found along a mile-long stretch of beach by Aquinnah, the area known as Gay Head. Coast Guard officials said an oil slick near the search area was unrelated to the crash.
On Saturday, investigators found a seat, a headrest, a nose wheel, several other airplane parts, a suitcase belonging to Lauren Bessette, and a prescription bottle belonging to Carolyn Bessette Kennedy.
Kennedy's six-seat Piper Saratoga II HP did not have black boxes, the devices required on all commercial planes that record cockpit conversations and radio transmissions as well as many parameters of an airplane's performance. However, Kennedy's plane did have a manual voice recorder, a device that is mostly unheard of on all but the most recent models of private planes.
The voice recorder would capture any radio transmissions and possibly some of Kennedy's utterances to his passengers, but would have had to be turned on by the pilot for it to operate.
It was unclear whether the recorder would yield much information. An official with the NTSB, which assumed control of the investigation yesterday, said a review of radio traffic showed that ground controllers received no communications or distress calls from the plane in the last 40 minutes it was known to have been in the air.
Still, investigators were hoping that Kennedy turned on the recorder before he taxied out to takeoff and that it might capture something that air traffic controllers never heard - a mayday call, or something that would reveal details of the final moments of the flight.
The NTSB assigned an investigator to track down the person who installed the device in an attempt to learn more about it.
Questions about the response time of the search effort were prompted by the actions Friday night of Adam Budd, a ramp attendant at the Martha's Vineyard Airport. Budd, 21, of Sharon, said he waited with a couple and their young daughter for the Kennedy plane to arrive.
After 10 p.m., the anticipated arrival time, Budd said, he checked with the tower but was told there was no sign of the plane and there was no indication that Kennedy had radioed a request to land.
Budd said he then telephoned Bridgeport Automated Flight Service Station in Connecticut, an FAA-run facility where pilots sometimes file their flight plans. Kennedy was not required to file a flight plan, and had not done so.
Budd said he talked to an operator at the Bridgeport station, whose toll-free number is well-known to area pilots.
''I told him the tail number [of Kennedy's plane], and I asked them if they had the tail number'' on a flight plan, Budd said. The operator was unresponsive, Budd said: ''He just didn't want to talk to me.''
George Mackie, an operations supervisor at the Bridgeport station, said yesterday that Budd's call should have triggered a search by the center.
Even if a pilot is flying under visual flight rules, as Kennedy was, and has not filed a flight plan, the flight service is expected to begin searching for an aircraft once it has been identified as late, Mackie said.
''If someone calls us and tells us an airplane is late, we will begin a search that usually starts with telephone calls,'' he said. The telephone inquiry usually consists of contacting small airports in the area, the FAA, and, eventually, the Air Force's Air Rescue Coordination Center in Virginia, Mackie said.
As it turned out, a search was not initiated until after a Kennedy family friend called the Coast Guard at Woods Hole at about 2:15 a.m. Saturday. The first search vessel was launched at about 4:30 a.m.
The FAA, which oversees the Bridgeport center, has declined to comment on the investigation and the response time.
Excitement that the plane might have been found was triggered yesterday when Coast Guard officials said a beacon signal was heard at about 3:30 p.m. by a huge Air Force C-130 that has been providing in-flight air control for the intensive air, water, and land search that began early Saturday.
The Coast Guard immediately dispatched the Rude (pronounced Rudy), a 90-foot ship belonging to the National Oceanic and Atmospheric Administration that is equipped with sonar capable of creating detailed images of objects beneath the surface.
Last night, however, Larrabee said the beacon signal appeared to have been a false alarm, possibly set off by a marker the Coast Guard had placed in the water to keep track of areas already searched.
An earlier beacon signal picked up before dawn Saturday off Montauk, Long Island, also was said to be unrelated to the search.
Larrabee said the search has touched nearly 9,000 square miles, but has focused primarily on a 364-square-mile area off Martha's Vineyard. The unusual objects identified by sonar were in an even smaller area, only 24 square miles.
The tent at the Kennedy compound where the Mass was said had been intended for 275 guests invited to the wedding of Rory Kennedy, the youngest daughter of RFK. John F. Kennedy Jr., 38, and his 33-year-old wife had planned to attend the wedding Saturday after dropping off Lauren Bessette, 34, on Martha's Vineyard.
The wedding was postponed indefinitely, and would-be wedding guests began trickling into Barnstable Municipal Airport in Hyannis yesterday to catch flights home. A friend of the Kennedy family said hope had dimmed by Saturday evening, and talk of survivors had been replaced by discussions of memorial services.
Inside the compound at Hyannis Port, where so many of the family's joys and tragedies have been played out, the Kennedys had no illusions about the news from Martha's Vineyard, family friends and associates said yesterday.
''They are doing as they always do: getting through this and dealing with it,'' said Robert Shrum, a friend and adviser of Edward Kennedy.
Pope John Paul II prayed yesterday for Kennedy and his family, telling aides ''this is only the latest tragedy that family has suffered,'' said papal spokesman Joaquin Navarro-Valls.
Nancy Reagan was among the list of notables who called with her condolences. President Clinton, speaking from the White House lawn, offered his prayers and said the Kennedys ''have suffered much and given more.'' Ethel Kennedy led a small group on a sailboat ride. Other mournful guests walked the white sand beach.
In Greenwich, Conn., the hometown of Carolyn Bessette Kennedy and her sister Lauren, about 200 friends, neighbors, classmates, and family members attended services at St. Michael's Church.
Also yesterday, NTSB officials gave the most complete account to date of Kennedy's fateful flight.
Robert Pearce, an NTSB regional director who is heading the Kennedy investigation, said the plane took off at 8:38 p.m. from Runway 22 at Essex County Airport in Fairfield, N.J. Kennedy had to make an immediate 180-degree turn to point his plane in a northeasterly direction.
He headed north at an altitude of 5,600 feet along the Connecticut coast. He passed over Westerly, R.I., at 9:26 p.m., at that same altitude. He then began his descent toward Martha's Vineyard, though Pearce said his precise rate of descent was not clear.
At 9:40 p.m., two radar stations at Cape Cod picked up the plane at 2,500 feet, 17 or 18 miles west of Martha's Vineyard Airport, which means he was about 10 or 11 miles off the island's coast.
During the next 29 seconds, the plane dropped fr om 2,500 to 1,800 feet. After that point - at 9:40:29 p.m. - the plane disappeared from radar.
Matthew Brelis, Tom Coakley, John Yemma, Cindy Rodriguez, Daniel Vasquez, Stephen Kurkjian, Ellen O'Brien, and John Aloysius Farrell of the Globe Staff contributed to this report.