A thorough investigation of the slaying a year ago U.S. Ambassador Francis Meloy Jr. [WORD ILLEGIBLE] an aide has reportedly convinced American officials that the assassination was a carefully planned left-wing terrorist operation that probably [WORD ILLEGIBLE] on an intelligence source inside the U.S. embassy here.

The motive of the attack - apparently by a Palestinian splinter group [WORD ILLEGIBLE] ties to Libya - is now believed to have been to provide the United States into a reaction that would have side-tracked efforts then under way to end the Lebanese civil war.

It is understood that the United States and Lebanon intend to make a point public statement soon of the [WORD ILLEGIBLE] of the U.S. investigation.

The American objective appears to be eventual Lebanese action against the suspects, who are believed to be [WORD ILLEGIBLE] in this country.

Meloy, his chief aide, Robert [WORD ILLEGIBL] and their Labanese driver were allied in an ambush last June 16 as they attempted to cross the "no-man's and" that divided wartorn Beirut.

The present U.S. ambassador, Richard Parker, will not publicly discuss the investigation and the affair is still considered highly sensitive by Labanese authorities.

According to several knowledgeable sources, however, the attack was the product of detailed planning and involved systematic surveillance, telephone monitoring and several contingency plans for exploiting expected opportunities to assassinate Meloy.

The Lebanese government of President Elias Sarkis has pledged support and cooperation in the effort to demonstrate Washington's intention to follow up on anti-American terrorism. There is no evidence here of any Lebanese action so far on the Meloy killing however, and it remains a subject officials are reluctant to discuss and apparently, unwilling to probe.

Unofficial evidence implicates a left-wing Lebanese splinter group known as the Socialist Labor Party - the Lebanese wing of an underground pan-Arab leftist movement headed by George Habash, who also is the leader of the Popular Front for the Liberation of Palestine.

The splinter group belonged to the Libyan-backed Alliance of Moslems and Palestinians. The alliance was battling against Lebanese Christians a year ago and opposed Syrian intervention in Lebanon's civil war, which enjoined American encouragement.

The Socialist Labor Party admitted the assassination at a closed-door session of the Moslem-Palestinian Coalition, according to accounts now circulating there. The murders were criticized by the meeting, but no punishment was meted out.

The Socialist Labor Party defended the operation as an act of "revolutionary violence." The small, violent group was linked with the kidnapings earlier in the war of U.S. Col. Ernest Morgan and two United States Information Agency officials.

For reasons of inter-Palestinian solidarity, Palestine Liberation Organization chief Yasser Arafat also refrained from acting against the terrorists. As a conciliatory gesture to the United States, however the PLO provided protection for the evacuation of American citizens ordered by President Ford three days later. To demonstrate it had nothing to do with the slayings, the PLO even handed over the undamaged embassy limousine to CIA investigators, Palestinian sources say.

Throughout the Lebanon war, Palestinian "rejectionists" pursued a policy of provocation, partly with the idea of creating a local revolutionary climate and partly to block any American-sponsored Arab-Israeli accommodation in the Middle East.

In addition, the U.S.-backed Syrian military intervention had started two weeks earlier, and the terrorists apparently preferred to strike at the Syrians through their American "allies" rather than risk conflict with the tough security forces of Syrian President Hafez Assad.

The sophistication of the assassination operation has also raised questions about possible backers for the Socialist Labor Party plot.

"It's too hot to discuss," a prominent Lebanese editor said this week, "But you might ask why the Libyan government has just been publiclly rebuffed by the Carter administration."

The U.S. State Department recently announced its rejection of a Libyan request for an exchange of ambassadors. Libya was named in a State Department list of Arab regimes accused of supporting terrorism.

Libyan support for the Socialist Labor Party and similar groups during the civil war here has been clearly established. At the time of the slayings Libyan Premier Abdel Salem Jalloud was furious over the failure of his attempt to pull off a proPalestinian settlement in Lebanon - a major political setback that he attributed to a Syrian-American alliance.

The operation clearly involved a considerable degree of skill. Incidents that initially appeared as lapses in embassy security - a failure of the trailing security car to follow orders, a malfunctioning of Meloy's car radio, and failure of Meloy's driver to take evasive action at a hostile roadblock - now are viewed as evidence of connivance by an ambassy employee.

The case is considered particularly poignant by Meloy's fellow diplomats because the ambassador undertook to cross the hostile "no-man's land" after he had received instructions from Washington to resume contacts with Lebanese officials on the other side of the divided, dangerous city.