The death toll in Friday's dual terror attacks on airports here and in Vienna rose to 18 today, including five Americans. Italian authorities disclosed that the attackers here apparently belonged to a previously unknown group who struck, at least in part, to avenge an Israeli air strike against PLO headquarters in Tunis in October.

The terrorists, in a note found in the pocket of the lone survivor of the four Arab gunmen who attacked Rome's Leonardo da Vinci Airport, identified the group as the Martyrs of Palestine. The group vowed to shed "rivers of blood" to avenge Israeli killings of Palestinians, including those slain during the air strike against the Palestine Liberation Organization headquarters. About 60 persons, both Tunisians and Palestinians, were killed in that attack.

The PLO has condemned yesterday's attacks and said it had nothing to do with them.

The toll in the Rome raid rose to 15 dead today when two wounded Americans died in Italian hospitals, bringing to five the number of Americans to perish, all in the Rome attack. The other victims were three gunmen whose nationalities are unknown, three Greeks, two Mexicans, an Algerian and an Italian. Three persons died in the Vienna attack -- one gunman, one Israeli and one Austrian.

Interior Ministry sources here said Don Maland, 30, of New Port Richey, Fla., died early this morning at St. Giovanni Hospital from wounds received in the shootout. Later, Elena Tomarello, a 67-year-old woman who had dual U.S. and Italian citizenship and had relatives in Naples, Fla., also died at the St. Camillo hospital, where she had been rushed immediately after the attack.

The other dead Americans were 11-year-old Natasha Simpson; John Buonocore, 20, of Wilmington, Del., and Frederick Gage, 29, of Madison, Wis. U.S. Embassy officials here confirmed that 12 other Americans were hospitalized as a result of the attack and another "seven or eight" were treated for minor injuries and released.

Revised figures issued today by the Italian Interior Ministry put the number of wounded at 74. Austrian authorities said 39 were wounded in the Vienna attack, a lower figure than was provided yesterday.

Austrian police said two Americans, whom they identified as Donna Kralik and Peter Lesley, were wounded at the Vienna airport.

The terrorists' note, written in Arabic and leaked in an Italian translation to journalists today by officials of the Interior Ministry, said the attack on Rome's sprawling airport was the opening salvo in a new war.

"The war has started from this moment," the note said. The attack with hand grenades and automatic rifles yesterday morning on the airport's check-in counter for TWA and the Israeli airline El Al came as three other Arab terrorists attacked the El Al ticket counter at Vienna's Schwechat Airport.

Although at least three little-known Palestinian terrorist groups have claimed responsibility for the attacks in anonymous phone calls to news organizations in Beirut, Vienna and Malaga, Spain, Italian security sources investigating the attack here said today that so far all they had to go on was the note found in the surviving terrorist's pocket and the fact they have identified him as a Palestinian.

The note was found in the pocket of the terrorist, identified as Mohammed Sarham, 19. He apparently intended for it to be found whether he lived or died in the attack.

"As you have violated our land, our honor, our people, we in exchange will violate everything, even your children to make you feel the sadness of our children," the rough Italian translation of the note read. "For every drop of blood shed, whether for the Tunis raid or other things, rivers of blood will be shed in exchange."

The mention of the "Tunis raid" was a reference to the long-distance Israeli Air Force bombing of PLO chairman Yasser Arafat's headquarters on the outskirts of Tunis Oct. 1.

Israel said the raid -- executed with midair refueling of fighters over the Mediterranean -- was in retaliation for the killing of three Israelis on a yacht near Cyprus. The gunmen said at their trial in Cyprus that the three Israelis killed were members of the Israeli intelligence organization Mossad, something the Israeli government has denied.

Sarham, who was shot in the chest and shoulder, was interviewed in his tightly guarded ward at the military hospital of Celio by Rome magistrate Domenico Sica, a specialist in terrorist cases. The gunman told investigators that he was born in the Sabra-Shatila refugee camp complex on the southern edge of Beirut. The camps were the scene of a massacre of Palestinians by Lebanese Christian militias in September 1982 when Israeli forces were occupying Beirut.

Police here said that two of the three gunmen who died in a hail of gunfire at the airport terminal had entered Italy on fake or stolen Moroccan passports, the same type that were used by Palestinian terrorists who bombed the Cafe de Paris and the British Airways office along the Via Veneto here in September.

Similiar phony Moroccan passports also were used by at least one of the four Palestinian terrorists who hijacked the Italian cruise ship Achille Lauro in October and two of the three hijackers who diverted an Egyptair jet to Malta last month. Moroccan passport holders, unlike many other Arab nationals, do not need visas to enter Italy.

Police officials said that two of the terrorists killed at the airport here had used passports with the names Mohammed Bou Darwish and Yasser Bou Hmida.

Police officials who declined to be named said today that although Bou Darwish's and Bou Hmida's passports had not been recovered in the raid, the two men using them had apparently been registered at two modest Rome hotels along the Via Cavour near the Coliseum as long as three weeks before the airport assault.

The police officials said that, according to receipts found in their pockets after they were slain, the two had checked in with the Moroccan passports to the Ferraro Pensione Dec. 6 and stayed there until the evening of Dec. 15. Then they paid their bill and moved down the avenue to the Pensione Cherie, where they stayed until checking out yesterday morning. They reportedly paid their bill in cash, and carried only overnight bags too small to have contained the Kalashnikov assault rifles they are accused of having used, along with Soviet-made hand grenades, to attack the airport.

Where the other two terrorists involved in the airport attack had stayed in Rome was not clear.

Police investigators here suspect that somewhere in or near Rome, Palestinian terrorists who may have supported the Cafe de Paris and British Airways bombings in September, and possibly other Italy-based Arab terrorist raids in recent months, maintain a secret "operational center" where weapons, false passports and money are provided to terrorists.

The Milan newspaper Corriere della Sera said investigators believe, based on preliminary examinations, that the terrorists were heavily drugged with amphetamines during the attack.

Since the bombing of the PLO headquarters in Tunis, dissident Palestinians, whom the PLO has officially disavowed, hijacked the Italian cruise liner Achille Lauro off the Egyptian coast and killed an elderly, wheelchair-bound American before surrendering to Palestinian and Egyptian authorities. The four hijackers are now in jail in Italy after U.S. Navy jets diverted an Egyptian airliner that sought to fly them to Tunis.

Following the embarrassment that hijacking provided the PLO because a member of the group's executive committee, Mohammed Abbas, was accused of having masterminded it, Arafat announced in Cairo last month that the PLO opposes all acts of terrorism outside Israel and the territories it occupies that the Palestinians still claim as their homeland.

Yesterday's attacks on the Rome and Vienna airports are seen here as one more action to discredit Arafat, who in recent years has been increasingly moving toward accepting the existence of Israel and negotiating with it.

It is this challenge to Arafat's leadership of the Palestinian movement that has given some credence here to the theory that the airport raids were the work of such renegade Palestinian hard-liners as Abu Nidal, who broke with Arafat in the early 1970s and has since spent more time fighting and assassinating PLO officials than confronting Israel.

[CBS television and radio later broadcast an interview with a man it said was one of the gunmen in the Vienna attack. The wounded man said from his hospital bed that his name was Mahmoud and that he and his two colleagues were members of the main Fatah branch of the PLO.]

The Chicago Tribune reported the following:

An airport security official said the high casualty toll in Rome was due to the fact that the terrorists threw two hand grenades into the hall's crowded coffee shop before firing bursts from their automatic weapons at passengers checking in at the El Al and TWA counters for flights to Tel Aviv and New York.

In the gun battle between terrorists and security agents, bystanders were killed or wounded in the cross fire.

In fact, a South American tourist, Annalisa del Grande, said she watched gun-toting security men run among the wounded and dying in search of the terrorists, who had taken refuge behind the bodies.

"One of the terrorists was lying near me. He was wounded but reached for his rifle as two Israeli security agents approached. One of them hit him on the shoulder with his pistol butt, but the terrorist still went for his gun. So the second agent shot him dead at point-blank range. The blood splattered all over my suitcase."

The fourth terrorist was captured by Italian security men as he tried to escape, limping from a wound in the leg.

He had to be hit on the head with a pistol to stop him from running, and the enraged crowd punched and pummeled him before he could be rescued by police reinforcements.

Italian actress Sandra Milo, who was flying to New York, said she saw the wounded terrorist "dragged along the ground by his hair" by the infuriated mob before he was rescued.