Reporters in Beirut have now had a week in which to investigate the story of the massacre. They have been interviewing survivers of the two Palestinian refugee camps, outside observers, members of the Phalangist forces who had entered the camps, Red Cross representatives, and Israeli officers and soldiers who were in the area.
These investigations have failed to turn up evidence of a substantial number of Palestine Liberation Organization military units or individuals.
One reason given for the advance of Israeli forces into Beirut in violation of the agreement negotiated by United States Ambassador Philip Habib was that Israel had evidence that 2,000 members of PLO units had been left behind and were still in west Beirut, presumably as a nucleus for rebuilding a PLO armed position in the area.
The other reason given was to maintain ''law and order'' following the assassination of Lebanon's newly elected president, Bashir Gemayel.
Instead of maintaining ''law and order'' the Israeli forces first pushed Lebanese regular army forces away from the two camps where the massacre occurred , then released into those camps the Phalangist and other Lebanese units which had been in Israeli pay.
Observers including US Embassy officials agree that probably there were a few PLO men hiding among the refugees. But, as reported earlier by the US mission in Beirut to the Israeli government, the only significant number of PLO people left behind when Yasser Arafat led his fighting men out of Beirut were members of the official PLO diplomatic mission and a few young men who had been assigned part-time police work in the camps.
Substantially, the PLO people who had remained were there with the consent of the government of Lebanon. Their continued presence was part of the arrangement made by Mr. Habib which led to the departure of the PLO fighting forces from Beirut.
In other words, the justification for the Israeli move was non-existent.
The allegation about the PLO was the latest in a series of less than candid reasons advanced by Israelis along the road which Israel entered upon with the invasion of Lebanon in June and which ended with the massacre in mid-September.
The first forward move of Israeli troops following the Habib accord and the cease-fire was a short advance into the village of Bir Hassan on Sept. 3, just two days after the departure of the last PLO units from Beirut. The excuse given was that the PLO had planted land mines in Bir Hassan. The move turned out to be the first step in the final surge into west Beirut.
A further reason for the move into Bir Hassan may have been to take attention away from President Reagan's peace plan which had been launched three days earlier. The decisive move later into west Beirut came on Sept. 15 and smothered further progress with the peace plan. American diplomacy had the new crisis to handle.
Then we go back to the beginning of the Israeli invasion of Lebanon. It was called ''Operation Peace for Galilee.'' US government officials were informed that the intention was to go only 25 miles into Lebanon in order to drive PLO forces beyond artillery range of villages in Galilee. The Israeli advance reached the 25 miles and kept going without hesitation.
As for the justification. The Israeli ambassador in London was attacked and wounded on June 3. British police found papers indicating that the attackers belonged to an anti-PLO group of Arabs. British Prime Minister Margaret Thatcher so informed Parliament and the Israeli government.
But on June 4 Israeli planes delivered a ''massive'' air attack on west Beirut, and Israel sent its ground forces forward on June 6.
The story really began nearly a year earlier, on July 17, 1981, when Israeli forces bombed the center of west Beirut killing some 300 and wounding an estimated 800 others. Ambassador Habib was sent at once to the area. He negotiated a truce which was declared on July 24.
The only violation of that truce by PLO forces came on May 9 when the PLO fired 100 rounds into Israeli territory. There were no casualties reported. This was in reprisal for two heavy Israeli air raids on PLO positions along the Lebanon coast, the first on April 21, the second on May 9. The truce was broken, not by the PLO, but by Israel.
The record is clear that Israel had been preparing the invasion of Lebanon from late 1981 and launched it when it was ready.
Arthur Hertzberg, a former president of the American Jewish Congress, wrote in last Sunday's New York Times, ''The most precious asset of Israel, its credibility, is now severely damaged. . . . Begin and Sharon must go.''