A Pan Am jumbo jet apparently exploded in the air before crashing and killing all 259 people aboard, and investigators focused Thursday on terrorist threats to place a bomb on a Pan Am airliner.

U.S. embassies had posted warnings that a bomb threat had been made on a Pan Am jet flying from Frankfurt to the United States sometime in December.The crash of the Boeing 747 bound for New York with Christmas travelers scattered bodies and wreckage over a wide area and ravaged Lockerbie village. The doomed Pan Am Flight 103 originated in Frankfurt aboard a different plane.

Twenty-two people died on the ground, most of them in cars hit by wreckage, said Desmond Park, deputy chief executive of the area's local council. He said the information was from other town authorities.

The plane smashed into the ground, destroying 40 houses, triggering an explosion at a gas station and engulfing cars in flames.

The death toll on the plane rose to 259 Thursday when it was discovered that another flight attendant was on board, said Pamela Hanlon, a Pan Am spokeswoman in New York. The victims included U.S. servicemen, 38 students from Syracuse University and an Associated Press executive and five of his family members.

Neither Scottish Secretary Malcom Rifkind, the chief British official at the site, nor U.S. Ambassador Charles Price would speculate on the possibility of sabotage, but both the International Air Transport Association and the British Airline Pilots Association said that was the most likely explanation.

In Washington, a U.S. government source said the U.S. Embassy in Helsinki, Finland, received a telephone threat about two weeks ago from a person claiming to belong to the Abu Nidal group, a radical Palestinian faction that has been implicated in terrorist attacks.

The caller said a bomb would be placed aboard a Pan Am plane and that it would be carried by a woman, said the government source, who spoke on condition of anonymity.

The government source said that threat was the basis for posting notices at U.S. embassies.

In Moscow, the U.S. Embassy last week warned American diplomats that a bomb threat had been made against a Pan Am flight flying from Frankfurt, West Germany to the United States sometime in December. The warning left it to the diplomats' discretion whether to change travel plans.

Press Association quoted intelligence sources as saying the tip came from Mossad, the Israeli intelligence service. "It is believed the U.S. received the warning with some circumspection but advised all its embassies in Europe to post an internal memo on their notice boards," Press Association said.

In Frankfurt, the airport security chief said officials stepped up security at Pan Am after receiving threats against the carrier earlier this month.

An anonymous male caller to The Associated Press in London claimed responsibility for the disaster and said it was in retaliation for the shooting down of an Iran Air Airbus by the U.S. Navy cruiser Vincennes in July. The Navy said the plane carrying 290 people over the Persian Gulf had been mistaken for an Iranian fighter.

"We the guardians of the Islamic revolution are undertaking this heroic execution in revenge of blowing the Iran air plane by America a few months ago and keeping the Shah's family in America. We are very proud," the caller said, then hung up quickly.

The group also claimed responsibility for a July 18, 1987, car bombing in London that wounded Amir Hussein Amir-Parviz, chairman of National Movement for Iranian resistance and a former Iranian cabinet minister under the Shah.

Iran's Prime Minister Hussein Musavi "categorically denied any Iranian involvement" in the crash. The denial came in a report by the official Islamic Republic News Agency, monitored in Nicosia, Cyprus.> Scotland Yard said the head of its anti-terrorism branch, Commander George Churchill-Coleman, had taken charge of the London end of the crash.> Transport Secretary Paul Channon told the House of Commons that searchers had recovered both the airplane's flight recorders, the vital electronic devices that monitor all flight data and conversation by the pilots.> Prime Minister Margaret Thatcher and Prince Andrew, the Duke of York, visited Lockerbie and talked to rescuers.

Rifkind, the British Cabinet minister responsible for Scotland, said the widespread wreckage of the plane indicated "the aircraft clearly experienced some form of explosion."

"It's clear that the accident happened in the air because parts of the wreckage are strewn over such a wide area," said Rifkind, who toured the crash site with Price.

Price also said the way the plane had split into several sections "would indicate a midair explosion."

Asked on NBC-TV's Today show about the reported bomb threats, Price said: "No, we did not have any specific information such as that in London."

When he was asked how many Americans were on board, he said he "understood that there were some 49 servicemen" on the jet.> Also on the jet was John Mulroy, 59, director of international comunications for The Associated Press.

Wednesday's crash, the worst in British history, occurred 54 minutes after takeoff from London's Heathrow Airport and followed an apparently routine climb to a cruising altitude of 31,000 feet, airline officials and traffic controllers said. The only cause ruled out was a collision with another plane.

Freddie Yetman, technical secretary of the British Airline Pilots Association, said only sabotage or structural failure could have destroyed the airplane at that height, and: "It seems to me that the possibility of structural failure is almost too remote to be considered."> "Sabotage is the most likely explanation," said David Kyd, chief public relations officer for the International Air Transport Association in Geneva. He said structural problems were unlikely to have been involved.> Police Superintendent Angus Kennedy told a news conference Thursday that the bodies of 150 passengers, including three children, had been recovered and were taken to a makeshift morgue in the town hall.> The wreckage was strewn over a 10-mile area, and it appeared the Boeing 747's nose fell to earth before the main part of the jet struck Lockerbie.> The plane's 243 passengers, many of them carrying Christmas packages, included three children, 38 undergraduates studying abroad on a Syracuse University program and the U.N. commissioner for Namibia, Bernt Carlsson of Sweden. The plane had a seating capacity of 412.

There also was a crew of 16 aboard the ill-fated plane.