A TWA spokeswoman in New York said most of the 114 passengers and seven crew members aboard the Boeing 727 were Americans, including the four people who were killed.
Three of the dead were adults, and the fourth victim was an 8-month-old girl.
At least nine other passengers, including three Americans, were injured when the bomb exploded inside the plane as it was en route from Rome to Athens and then Cairo, Egypt, officials said. They said the bomb had been placed, possibly in a piece of hand luggage, under a seat in the 10th row of the aircraft.
U.S. officials in Washington speculated that the bomb could have been made
from plastic explosives, which would not have shown up when hand luggage was X-rayed before being carried on board.
"It was definitely an explosive device," a Greek government spokesman said, calling the planting of the bomb a "barbarous terrorist action."
The U.S. State Department said it could not yet say definitely whether the bomb was a terrorist act. President Reagan, vacationing in Santa Barbara, Calif., ordered U.S. authorities to cooperate in the investigation into the cause of the explosion, White House spokesman Larry Speakes told reporters. Officials in Washington said that the Justice Department, the FBI and the Federal Aviation Administration had been assigned to the task.
The bomb exploded shortly after 2 p.m. (6 a.m. Philadelphia time). Several hours later, a caller who said he spoke for a pro-Libyan Palestinian group claimed responsibility for the attack.
In telephone calls to two Western news agencies and to a newspaper in Beirut, Lebanon, the man said the Kassam Unit of the Arab Revolutionary Cells was behind the attack. He warned of more "revolutionary action to strike American imperialist interests everywhere."
The man, speaking in Palestinian-accented Arabic, said the attack was also in retaliation for "all American imperialist attempts to bring our Arab masses to their knees . . . the last of which were the failed attempts to attack Libya."
The bombing came only days after Libyan leader Moammar Khadafy had urged attacks on U.S. government and commercial targets in retaliation for U.S. attacks on Libyan military targets during the Gulf of Sidra dispute last week.
In an interview at his compound on the outskirts of Tripoli, however, Khadafy denied any involvement in the TWA explosion.
"I am completely against any action like this. This is an act of terrorism against a civilian target, and I am totally against this," he said. "And, of course, I reject this. We do not attack civilian targets."
TWA flight attendants are on strike against the airline, but company officials emphatically ruled out any link between the bombing and the labor dispute.
In Washington, the chairman of the House Foreign Affairs Committee and a prominent expert on terrorism - both speaking before the claim of responsibility was made - said they believed that the bombing might have been related to Khadafy, but neither suggested that the Libyan leader ordered the attack.
Terrorists may have set the bomb "to make us look impotent," said Robert Kupperman, an expert on terrorism at the Georgetown University Center for Strategic and International Studies.
"My instincts tell me that this is related to the Gulf of Sidra matter," said Kupperman. "In my mind, I see it as purely a retaliation."
Rep. Dante B. Fascell (D., Fla.), the committee chairman, said that the incident, "coupled with the very real possibility of Libyan-inspired terrorist attacks following our recent freedom-of-navigation exercise in the Gulf of Sidra, underscores the need for prompt and effective action to combat the spread of international terrorism."
Fascell said the Foreign Affairs Committee was planning to review the bombing during hearings this month.
In New York, TWA president Richard Pearson said the pilot of Flight 840, Capt. Richard Petersen of Sarasota, Fla., landed the plane at the Athens airport despite the hole in the plane's right side.
There was no panic among the 121 on board, said Pearson, who told reporters in New York that the landing was "normal, routine" and "very uneventful . . . thank God."
"Everyone was strapped in," TWA spokesman Dann Oldani said. "The no- smoking signs were on. That's standard procedure at 15,000 feet."
After the blast "the plane stayed up in the air because it was flying at a relatively low altitude so there was little difference in pressure between the cabin and outside," said a senior Athens airport security official, Panagiotiso Christopoulos.
An explosion at a higher altitude would have caused more severe decompression and structural damage, possibly leading to a crash, aviation experts said.
Airline officials said the bomb apparently was on the floor of the passenger cabin under seat 10F. Police sources in Greece said the bomb probably was carried aboard the aircraft in a passenger's hand luggage.
After the plane landed, police detained all the passengers - many of them shocked and exhausted - for several hours while they questioned them about the incident.
Police sources said officers were continuing to question five Arabs, including two Lebanese nationals, who were aboard the plane, and were also investigating the possibility that the blast was the work of a suicide bomber.
However, authorities at Leonardo da Vinci Airport in Rome were investigating whether explosives were smuggled onto the plane before passengers boarded in Rome.
Flight 840 originated on a Boeing 747 in Los Angeles Tuesday morning and stopped in New York before proceeding to Rome, a TWA agent said. Retaining the same number, the flight changed to a Boeing 727 in Rome for the next legs to Athens and Cairo.
Airline officials said the 727 had arrived in Rome yesterday morning from Cairo and Athens.
As a routine security matter, Pearson said, Italian inspectors both X-ray and hand-search each piece of carry-on luggage brought aboard any plane leaving Rome. This, he said, also was done for luggage brought aboard the 727.
Authorities said, however, that the checked baggage of the passengers who arrived from Los Angeles was not screened again before being transferred to the 727.
The plane's flight data recorders, the so-called black boxes, were expected to be sent to the United States today for review, state-run Greek television said.
Police in Greece said three bodies were found in the countryside 120 miles southwest of Athens, and the fourth was later found in the sea nearby.
Reports from friends, relatives, Greek officials and TWA identified the victims as Alberto Ospina, 37, a Colombian-born American from Stratford, Conn.; Demetra Stylian, 52; her daughter, Maria Klug, 25, and granddaughter Demetra Klug, 8 months old, all from Annapolis, Md.
Police sources identified the injured Americans as Henry Siemsen, 70, a retired U.S. Army colonel; his wife, Myrtle Siemsen, 67, and James Carlton Denon, an American whose age and hometown were not immediately known.
The four injured Greeks were identified as Panayotis Karasavas, George Matessis, Despinia Hioti and Eleni Phillips.