The investigation into the assassination of Lebanon's president-elect Bashir Gemayel in September has led Phalangist Party officials to conclude that Syrian intelligence almost certainly was behind the killing. But the inquiry results may never be published--largely because the key suspect escaped but also, apparently, for political reasons.

Phalangist officials have told several western embassies and reporters that the man manipulating the captured suspected assassin, Habib Shartouni, was a member of the Lebanese National Syrian Social Party and they have identified him as Nabil Felaghi, also known as Nabil Alam.

Alam, according to these officials, was one of four party leaders who resigned Oct. 5, three days after the capture of Shartouni was made public.

The National Syrian Social Party is one of the 13 groups that made up the now-defunct leftist National Movement, and the party has long had close ties with the Syrian government because of its adherence to the doctrine of a "natural Syria" incorporating all of Lebanon.

The Phalangist Party has never publicly accused Syria of involvement in the assassination.

Its officials say this is because they did not succeed in capturing Alam to confirm Shartouni's testimony about the Syrian link.

They also indicate, however, that nothing has been said about the Syrian role because it could complicate the Lebanese government's efforts to gain Syrian cooperation for the withdrawal of its troops from Lebanon together with those of Israel and the Palestine Liberation Organization.

Alam fled Lebanon before he could be seized and Phalangist officials believe he is living in Syria. He reportedly was identified as the mastermind behind the plot by Shartouni, whom the Phalange captured a few days after Gemayel's assassination Sept. 14. Shartouni reportedly was trying to make his way across the mountains northeast of Beirut to Syrian lines.

On the basis of testimony taken from Shartouni, Phalangist officials say, Alam was linked to the Syrian and Palestinian intelligence services and had spent six months in the Soviet Union for training.

But they believe he was acting primarily on behalf of the Syrians, who, they point out, had been attacking Gemayel in the state-controlled media since his nomination as the presidential candidate of the Phalange-led Lebanese Forces in August.

The Syrians and Phalangist Party had long been at odds and fought each other in northern Lebanon and East Beirut throughout much of 1978 and during the Syrian siege of Zahle, in the Bekaa Valley, in 1981.

The enmity between Syria and Bashir Gemayel was strong, particularly after his secret ties to Israel became known, and observers say this may help to explain the Phalangist inclination to suspect Syrian involvement in the assassination.

Initially, most Phalangist officials suspected Israel because of a confrontation Gemayel and Prime Minister Menachem Begin had in Nahariyya, Israel, in early September over Gemayel's refusal to agree to sign a peace treaty with Israel immediately after he took power.

Syria switched its policy and propaganda toward the Phalange after Gemayel's death and the election of his brother, Amin, to replace him as Lebanon's next president.

The Syrians had always gotten along better with Amin, who was far less opposed to Lebanon's ties to Syria and the Arab world generally than was Bashir.

As a gesture of good will following Amin Gemayel's election, the Syrians even released a half dozen Phalangist officials they had been holding for several years and also sought to improve relations with the new Lebanese government.

The Lebanese Forces are holding Shartouni in a prison in East Beirut and say he will be handed over to the central government for a public trial in due time.

Shartouni's grandfather was the landlord of the house in which Gemayel was killed by a huge bomb explosion while addressing Phalangist Party members.

According to Phalangist officials, this explains how it was possible for Shartouni to penetrate Gemayel's security and place 77 pounds of TNT in an empty room above the hall where the Lebanese Forces leader was speaking.