Antiterrorist police investigating the attempted bombing of an El Al jumbo jet here last Thursday are "100 percent satisfied" that it was unrelated to a U.S. air attack against Libya two days before, sources here said today.

Investigators have determined that the El Al bombing was plotted long before the raid on Libya. While they believe it was planned under the auspices of a terrorist organization allied with at least one Middle Eastern country, sources said police are not yet ready to say which organization, or which country.

Libyan involvement has not been ruled out, nor has a role by Syria, which supports a number of such organizations in the Middle East.

Much of the information police have gathered so far on the El Al plot has come from Nezar Hindawi, a 31-year-old Palestinian of Jordanian nationality who was arrested Friday. Yesterday, Hindawi was formally charged with conspiring to murder the passengers aboard the Tel Aviv-bound jet. The jet was only a half hour from take-off at London's Heathrow Airport when a bomb he allegedly concealed in the luggage of his unsuspecting girlfriend was discovered.

During his first court appearance yesterday, police testified that Hindawi had entered Britain illegally, and that a passport bearing a "false identity" had been found in his possession. The owner of the hotel where Hindawi was found said he carried a Syrian passport.

According to the formal charges against him, Hindawi conspired on the bomb plot for two months "in London and elsewhere," with unnamed "others."

Sitting quietly in the heavily guarded courtroom, unshaven and dressed in an open-necked shirt and slacks, he appeared weary and a bit dazed.

Although a tip from Scotland Yard led West German police last weekend to arrest Hindawi's brother, Ahmed Nawat Mansur Hasi, for alleged complicity in the April 5 bombing of a West Berlin discotheque, that information was transmitted while Hindawi was still a fugitive.

The call from Scotland Yard to West Berlin came from efforts to find Hindawi, rather than any initial linking of the two crimes.

Another brother, Mahmoud Hindawi, has been living here for at least 20 years, employed by various Middle Eastern embassies and living with his own family in a house near Heathrow Airport, according to neighbors and acquaintances. It was he who persuaded Nezar Hindawi to turn himself in to police Friday.

Mahmoud Hindawi is not believed to have been involved in the alleged activities of his two brothers, sources said and was taken into police custody for his own protection.

Only little is known about Nezar Hindawi -- based on police comments, reports from Jordan and conversations with Arabs in London who claim to know him.

If Hindawi is ultimately found guilty, he may turn out to be a not-very-bright young man who acted out of naivete, self-promotion or misguided zeal for more sinister people. Alternatively, evidence may be found that he is a clever poseur, who plotted and bided his time for years under cover of another identity.

Nezar Hindawi traveled within the relatively large Jordanian community here and drifted in and out of Britain for several years. While acquaintances described him as "anti-Zionist," none believed him to be politically active or interested.

Sources in Jordan, who did not want to be identified, said he is thought to be a member of the Hindawi family which can be traced back to a larger clan named Hasi. The clan may have come to Jordan before 1948, during the Palestinian exodus.

Hindawi is known in Amman, Jordan, as a part-time journalist. His reports from Britain and Western Europe have included stories about AIDS and how rich Arabs from the Persian Gulf region spend their money in England.

One Amman source said Hindawi seemed more interested in stylish clothes and furthering his journalistic endeavors than in political matters.

According to Mohammed Kabarday, the Jordanian editor of the Arabic language newspaper Al Arab here, Hindawi applied for a job in 1982, saying he had worked for newspapers in the Middle East. Doubting his credentials, Kabarday hired him as a copy aide and sometime sportswriter.

Kabarday said he fired Hindawi after 2 1/2 months for antagonizing other staffers, and for locking the teleprinter room and leaving with the keys after a fit of temper.

About one year ago, Hindawi began dating Anne Marion Murphy, 32, an Irish citizen who worked as a chambermaid. She reportedly became pregnant with his child. Several weeks ago, Murphy traveled to her family home outside Dublin and, according to her parents, told them of her pregnancy and plans to be married. They had not met Hindawi.

Unconfirmed accounts here and in Jordan have said that Hindawi already was married to a Polish woman who now lives in her native country, and that she has a 4-year-old daughter with him.

Last Thursday, Hindawi dropped Murphy off at Heathrow Airport for her El Al flight to Tel Aviv where they were to wed. Israeli security personnel discovered 10 pounds of plastic explosive in her luggage.

Police concluded she had been unaware of the concealed bomb. Her statements led investigators to a residential hotel in West London. Hindawi had checked out that morning.

On Thursday afternoon, police distributed his photograph. The next day, a hotel owner, Naim Oran, received a telephone call that led him to Hindawi.

Oran said he visited Hindawi, and then telephoned his brother, Mahmoud, whom he knew. The brother then persuaded Hindawi to give himself up to police.