Undated (AP) _ Greek police sources said today they are hunting for a suspected woman terrorist they say was on a TWA jet hours before a bomb exploded on the aircraft, tearing open the cabin and killing four Americans.

Earlier, Italian officials said a known Arab terrorist occupied the seat of the TWA jet where the bomb exploded.

Italian Interior Minister Oscar Luigi Scalfaro told reporters in Rome, ''It is certain that a suspect person, who is on file as a terrorist, got on in Cairo and got off in Athens, occupying in the airplane the exact seat where the explosion occurred.''

The Boeing 727 flew Wednesday from Cairo, Egypt, to Athens, Greece, and then to Rome. There it picked up 112 passengers and headed back to Athens as TWA Flight 840, ultimately bound for Cairo.

The bomb exploded as the jetliner approached Athens airport from Rome, and the four victims were sucked out of the plane, flying at about 15,000 feet.

Greek police sources identified the suspect as a woman, May Elias Mansur, who may have passed through Greece previously.

''We have launched a search around Athens and other cities and also put out a signal to trace this person through Interpol,'' one source said.

In Rome, the Italian news agency ANSA tonight quoted unidentified Italian investigators as saying the women sought was believed to have boarded with a Lebanese passport.

ANSA said the woman may have boarded a Beirut-bound Middle East Airlines flight from Athens Airport shortly after arriving there.

An Egyptian security official at Cairo International Airport said that after the explosion, Egyptian authorities checked the names of passengers who boarded in Cairo and ''we had no suspicion about anyone on the list.'' The official spoke on condition of anonymity.

In other developments today:

-Palestinian sources in Beirut said a little-known group that claimed it staged the attack, the Arab Revolutionary Cells, was associated with Abu Nidal, whose group was blamed by the United States for Dec. 27 terrorist attacks at the Rome and Vienna airports. Twenty people died in the attacks, including five Americans.

-In Athens, TWA experts, U.S. Federal Aviation Administration and FBI officials, and investigators from Greece and Italy began examining the jetliner to determine if the bomb exploded in the cargo bay or in the cabin.

Greek officials said Wednesday that the blast took place in the cargo hold, but TWA officials in New York said it was in the cabin.

The pilot, Capt. Richard Peterson, told reporters today at Athens Airport, ''The explosion was above the floor. It looked like it must have been beneath a seat.''

Athens airport officials said the plane's ''black box,'' which records conversations between the pilot and the control tower, would be flown to New York for study.

The blast blew a 9-by-3-foot hole in the side of the plane in front of the right wing. TWA President Richard D. Pearson said in New York the explosion occurred on the cabin floor at row 10 or 11 of the passenger seats.

Officials at Rome's Leonardo da Vinci Airport said the terrorist was an Arab who sat in seat 10F on the Cairo-to-Athens flight.

ANSA said an Arab named Mansur or Monsour was sitting in the 10th row on that flight.

An anonymous caller to Western news agencies in Beirut claimed responsibility for the TWA attack on behalf of the Arab Revolutionary Cells. He said it was in response to last week's clash between U.S. and Libyan forces in the disputed Gulf of Sidra.

The Palestinian sources in Beirut, who spoke on condition of anonymity, said Abu Nidal had used the name Arab Revolutionary Cells in previous terrorist attacks.

Abu Nidal, whose real name is Sabry al-Banna, said in a statement issued in Damascus, Syria, last week that his group would strike at U.S. targets in retaliation for the U.S.-Libyan confrontation.

The Reagan administration also has accused Libya of backing Abu Nidal's group in the December airport attacks.

Libyan leader Col. Moammar Khadafy, however, distanced himself from the TWA attack, saying, ''This is an act of terrorism against a civilian target, and I am totally against this,'' CBS News reported.

Reagan administraton analysts believe Libya is not responsible for the explosion, a senior administration official in Washington said today. However, the undersecretary of state for political affairs, Michael Armacost, told a Washington news conference that possible involvement by Khadafy could not be ruled out until an investigation was completed.

After last week's confrontation in the Gulf of Sidra, during which the United States said at least two Libyan patrol boats were sunk and a missile site attacked, Khadafy threatened to attack U.S. interests worldwide.

The Greek government issued a statement saying it ''condemns the barbarous terrorist action ... and reiterates that terrorism undermines peace and democracy.''

The statement said the blast was caused by an ''explosive device,'' but it did not specify what kind. Greece's undersecretary for foreign affairs, Yiannis Kapsis, said the explosion was caused by a bomb in a piece of luggage.

Among the four Americans who died when they were sucked through the hole in the fuselage were a mother, daughter and baby granddaughter.

Reports from Greek officials, TWA, friends and relatives identified the victims as: Alberto Ospina, a Colombian-born American from Stratford, Conn.; Demetra Stylian, 52; her daughter, Maria Klug, 25, and 8-month-old granddaughter Demetra Klug, all from Annapolis, Md.

Seven other people, including four Americans, were injured aboard the plane, which landed safely in Athens 10 minutes after the explosion. Three of the injured were hospitalized overnight and released this morning, said the chief nurse at Voula Hospital near the Athens Airport.

Rome airport officials said 101 of the plane's passengers had transferred from a connecting flight from New York. A TWA official said they and their hand luggage had been checked in Rome but that their checked baggage, which was in sealed containers, was not.

The other 11 passengers began the flight in Rome. The airliner said besides the passenges, the plane carried seven crew members and three off-duty TWA employees.

Last June, a TWA jetliner flying from Cairo to Rome via Athens was hijacked to Beirut by gunmen who killed a U.S. Navy diver on board.