The body of the elderly man that washed ashore in Syria has a bullet hole in the head, U.S. officials said yesterday, and one Justice Department aide said there is a "strong presumption" that the body is that of Leon Klinghoffer, the Achille Lauro passenger killed during last week's hijacking.

The U.S. officials stressed they were awaiting a positive medical identification that the body was Klinghoffer, the 69-year-old invalid New Yorker shot to death aboard the Italian cruise ship last week. Officials said the body was missing both legs and one arm, with one calling it "very grotesque."

If the body is confirmed to be Klinghoffer, it would contradict assertions by the Palestine Liberation Front (PLF), which conducted the hijacking, that its members did not shoot Klinghoffer and that he had suffered a heart attack. One ship passenger said yesterday that he had heard shots and that a crew member told him that he had seen one of the terrorists shoot Klinghoffer in the head and chest.

U.S. officials said they expect a positive identification. In that part of the world, one said, "There aren't a lot of reports of elderly men floating around with a bullet hole in their left forehead."

Federal prosecutors, meanwhile, plan to present evidence to a grand jury here Thursday about the four Palestinian hijackers and PLF leader Mohammed Abbas, whom U.S. officials have accused of masterminding the hijacking, according to sources.

The case will be taken before the grand jury that is investigating the hijacking of a TWA airliner in June. Sources said any indictment against the ship hijackers -- which could be returned in a matter of days -- would disclose evidence from a sealed government affidavit that is said to rely heavily on intercepted radio communications between Abbas and the four Palestinians aboard the Achille Lauro.

A Justice Department official said details of the affidavit could not be made public before possible grand jury action because the Reagan administration does not want to prejudice any legal proceeding against the hijackers. He said the department had to keep the evidence under seal, even though Abbas has been declaring his innocence and Italian authorities contend there was not enough evidence to arrest him.

This official said the evidence in the affidavit was not ambiguous and that U.S. investigators are gathering more information that ties Abbas to the hijacking.

Abbas was captured Thursday night with the four hijackers when an Egyptian airliner on which they were flying was forced to land on the island of Sicily by U.S. Navy jet fighters. The FBI is heading a worldwide search for Abbas, whose whereabouts are unknown. The four hijackers have been charged in Italy and are not likely to be extradited to the United States, where they and Abbas have been charged in arrest warrants with hostage-taking, piracy and conspiracy.

One official said yesterday there was some confusion about whether the United States or Italy would be given custody of the corpse that washed ashore near the Syrian port of Tartus. But State Department spokesman Charles Redman said that Syrian authorities "are clearly prepared to turn the body over to us" if it is identified as Klinghoffer's.

Meanwhile, a Philadelphia judge who was among the Achille Lauro hostages held a news conference yesterday to describe again the circumstances of Klinghoffer's death, saying the ship's bartender told him that he had seen one of the hijackers shoot Klinghoffer.

Philadelphia Common Pleas Court Judge Stanley L. Kubacki said the ship's chief bartender, an Italian crew member, had told Egyptian authorities of Klinghoffer's murder 12 hours before Egypt allowed the hijackers to leave aboard the plane that was later intercepted by U.S. warplanes.

Kubacki said the hijackers took Klinghoffer in his wheelchair to the starboard side of the ship on the morning of Oct. 8. Ten minutes later, he said, he heard two shots from that part of the ship. Kubacki said he then heard a "double splash" as Klinghoffer and then his wheelchair hit the water.

Throughout the ordeal, the judge said, only one hijacker, whom he dubbed "the good terrorist," allowed the hostages to go to the bathroom. But, Kubacki said, he later discovered that "the good guy, the one that let us go to the toilet, was the one that put the bullet in Klinghoffer's head."

"They're such a danger to the world," he said. "What terrorists do is to take innocent, unarmed, helpless people, who have nothing to do with the question, and make them their victims. Their purpose is to shock the world with their outrageous behavior so that the world cringes."

Attorney Jay Fischer, a friend of the Klinghoffer family, said yesterday that the family had retained him to explore the possibility of filing a lawsuit in connection with Klinghoffer's murder.

Letty H. Simon, another friend of the family, said the family is in mourning and does not plan to attend a memorial service for Klinghoffer that will be held at 12:30 p.m. today at the U.S. Capitol.

Meanwhile, Knight-Ridder reported:

White House and Pentagon officials admitted yesterday that President Reagan and Defense Secretary Caspar W. Weinberger used an open air-to-air telephone line last Thursday to discuss a secret plan to intercept the Egyptian jetliner carrying the hijackers because they did not want to wait for antieavesdropping equipment to be rigged up.

The president and Weinberger were overheard by a ham radio operator discussing the top-secret mission more than three hours before it occurred.

Officials said the equipment on Air Force One was not compatible with gear on Weinberger's plane, and "a few minutes" would have been required for a special connection. "The question was, do a few minutes matter? . . . They decided to go ahead with the call because they felt time was critical," White House spokesman Michael Guest said.

An amateur radio operator who regularly monitors Air Force One transmissions heard enough to know the basic details of the upcoming interception. The operator called his brother, who then called CBS News. The network did not broadcast the information.