John F. Kennedy Jr. and his wife Carolyn Bessette Kennedy INTERACTIVE: The ship Rude, equipped with a special sonar, searches off the Massachusetts coast for John Kennedy Jr.'s lost plane RELATED VIDEO CNN's Kathleen Koch profiles John F. Kennedy Jr. Windows Media 28K 80K CNN's Richard Roth reports on Carolyn Bessette Kennedy Windows Media 28K 80K CNN's Jonathan Karl reports on the life of John F. Kennedy Jr. Windows Media 28K 80K CNN's Daryn Kagan provides insights on the life of John F. Kennedy Jr. Windows Media 28K 80K To view the latest video streams, use the arrow buttons below to scroll through the video gallery.        Windows Media Real        28 K 80 K RELATED AUDIO: Clinton expresses his support to the Kennedy and Bessette families (July 18) 218K/20 sec. AIFF or WAV sound  SPECIAL: Remembering John F. Kennedy Jr.   • Profile   • Pictorial Biography   • In his own words   • Flight Path and Search Area   • The Kennedy Tragedies   • Reactions  ALSO: Developments: Kennedy compound: Family keeps vigil, as crowds gather Bessette family tries to guard privacy amid public tragedy Mourners leave flowers, poetry on JFK doorstep The flight: JFK Jr.'s routine flight goes awry Reaction: World's eyes again drawn to Kennedy family drama Background: The Dynasty: TIME 100 Looks at the Kennedys Side-scan sonar takes wide survey of ocean floor MESSAGE BOARDS: Kennedy tragedy

In this story: More vessels joining search Beacon signal no lead after all Clinton offers tribute Debris being cataloged RELATED STORIES, SITES

MARTHA'S VINEYARD, Massachusetts (CNN) -- With almost no hope of finding anyone alive, ships and divers off Martha's Vineyard were to launch an intensive search Monday for the wreckage of the plane that was carrying John F. Kennedy Jr., his wife and her sister.

Searchers were interested in "a couple of targets" spotted by sonar 60 feet to 80 feet beneath the Atlantic Ocean surface southwest of the Massachusetts island.

But the Coast Guard stressed they're "simply potential targets" and not necessarily plane wreckage. The Piper Saratoga II HP vanished on a flight from New Jersey to Martha's Vineyard on Friday night.

Coast Guard Rear Adm. Richard Larrabee said Sunday that given the time that has elapsed since the plane disappeared from radar, search officials have concluded it is unlikely that Kennedy; his wife, Carolyn Bessette Kennedy; or his sister- in-law, Lauren Bessette, survived the crash.

Larrabee said he spent painful moments with the Kennedy and Bessette families late Sunday, telling them that hope had run out, and that the focus was being shifted away from rescue of any survivors to recovery of wreckage.

"It was very difficult to share this information with them," he said. "We offer our condolences to the families and loved ones."

In water temperatures of about 68 degrees, it is unlikely that anyone could survive longer than 12 hours, Larrabee said.

"We know that the aircraft was not equipped with any survival equipment -- life rafts, life jackets -- anything of that nature."

More vessels joining search

According to officials, recovering the plane's wreckage could take up to two weeks.

Three vessels searched the coastal waters overnight, according to Coast Guard Petty Officer Ed Cygan -- the Rude (pronounced "Rudy"), a sonar-equipped National Oceanic and Atmospheric Administration ship, and two Coast Guard cutters, the Willow and the Sanibel.

On Monday afternoon, the USS Grasp -- a Navy ship with extensive search-and-rescue capability -- is scheduled to arrive in the area from Norfolk, Virginia.

It carries remote-controlled underwater vehicles, called ROVs (remote-operated vehicles), Cygan said, and a crew of deep- water divers.

Also on Monday, a side-scanning sensor will be installed on the Willow, according to Coast Guard Petty Officer Michael Labine. The sensor, shaped like a torpedo, can examine a swath of ocean bottom 600 feet wide as it moves slowly along.

Beacon signal no lead after all

On Sunday, a U.S. Air Force plane detected a brief signal from what was thought to be an emergency beacon. But that lead didn't pan out upon further investigation, Larrabee said. He said a data marker dropped off by searchers may have been what was detected.

Kennedy's plane carried an emergency transmitting beacon that should have activated if the plane crashed, although the aircraft's manufacturer said the beacon might not work if submerged.

While new debris, including a headrest and insulation from an airplane, were recovered Sunday, searchers found no signs of survivors.

Meanwhile, top officials from the National Transportation Safety Board arrived on Cape Cod to begin a full-scale investigation into the crash. NTSB Chairman Jim Hall said the probe could take six to nine months.

"We are at the very beginning stages of what will be a painstakingly detailed investigation," he said.

Clinton offers tribute

After returning to the White House from a weekend at Camp David in Maryland, President Bill Clinton offered the sympathies and prayers of the nation to the Kennedy and Bessette families.

"At this difficult moment, we hope the families of these three fine young people will feel the strength of God, the love of their friends and the prayers of their fellow citizens," Clinton said.

He also offered a tribute to the Kennedy family, faced with the latest in a string of tragedies.

"For more than 40 years now, the Kennedy family has inspired Americans to public service, strengthened our faith in the future and moved our nation forward," he said. "Through it all, they have suffered much and given more."

Clinton said John F. Kennedy Jr. and his wife had "captured our imagination and won our affection."

Larrabee said it was difficult to tell the Kennedys the search had shifted from rescue to wreckage recovery

Debris being cataloged

On Sunday, Larrabee said a headrest from an airplane was found. Another headrest washed ashore Saturday.

Also, Capt. Robert Bird of the Massachusetts State Police said that insulation, consistent with the type used to line the aircraft cabins, washed up near Philbin Beach on Martha's Vineyard.

On Saturday, other debris -- including Lauren Bessette's suitcase and a prescription pill bottle bearing Carolyn Kennedy's name -- was found in the same area.

All of the recovered debris was being taken for cataloging to a Coast Guard facility at Wood's Hole on Cape Cod and will eventually be taken to Otis Air National Guard Base, where a command post for the search effort was set up Sunday.

Larrabee said the unified command post would oversee both search and recovery efforts, and include representatives from the Coast Guard, Navy, Air Force, Civil Air Patrol and NTSB.

John Kennedy Jr., 38, publisher of George magazine and son of the late president and Jacqueline Kennedy Onassis, bought the plane in April. He was a licensed private pilot.

He and Carolyn Bessette Kennedy, 33, whom he married in 1996, were on their way to Hyannisport, Massachusetts, to attend a Kennedy family wedding. They were to drop off Lauren Bessette, 35, a New York investment banker, on Martha's Vineyard before continuing on to Hyannisport.

Correspondent Carl Rochelle contributed to this report.

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