US tries to determine why hostage released. Evidence of Palestinian involvement in hostage holding is said to be growing

The Reagan administration is pleased with the release of an Indian hostage in Lebanon. But United States officials and foreign anti-terrorism specialists are not sure what it means for the remaining hostages or who was behind it.

Informed specialists presume Iran played a major role in securing Monday's release of Dr. Mithileswhar Singh. But several say they have less evidence of direct Iranian influence than in earlier releases.

Syria played a secondary role, well-placed experts say, though Damascus is trying to claim as much credit as possible.

Specialists are carefully investigating indications that Palestinians have assumed a greater role in controlling the three American professors taken along with the Dr. Singh in January 1987. They will be thoroughly debriefing Singh to try to shed light on the captors.

Washington adamantly denies any deal to secure the release.

``We've said it from the President down,'' says L. Paul Bremer, US ambassador-at-large for counterterrorism. ``And it's the fact - there is no deal and we're not going to do a deal.''

Iran's foreign minister Monday said his government had used its influence in the release. ``OK, let's see those with influence use it again and get all the hostages out,'' Ambassador Bremer says.

The same message is being passed to Iran through indirect diplomatic contacts, Bremer adds. ``It doesn't serve anybody's interest to have the hostages held. The hostage holders aren't going to achieve their aims and it can only be a burden in our relations. So why not get them out?''

Bremer and other officials say Monday's release combined with positive signals from Tehran could bode well for other hostages, but it's too early to tell. Washington has no indications that any Americans will be released soon.

Experts are puzzling over evidence surrounding the release of Dr. Singh and the status of the remaining Beirut University professors.

First, specialists say, there is increasing evidence of a Palestinian role in this affair.

Israeli experts have long said the kidnapping was a rare joint operation of elements from the Lebanese Shiite Hizbullah and Al-Fatah, Yasser Arafat's branch of the Palestine Liberation Organization (PLO).

United States and European specialists are skeptical that Fatah would involve itself in taking US hostages. But recent intelligence from Lebanon suggests the hostages may have changed from Hizbullah to Palestinian hands.

The last four communiqu'es from the hostage holders also reflect a clear Palestinian twist, evident when the professors were first taken. For example, the group demanded United States support for the Palestinian uprising to win the professors' release. Singh was reportedly asked to make a pro-Palestinian statement after his release.

Experts aren't sure what this means. Iran may still have played a big role. The Palestinian twist may be misinformation, giving Hizbullah deniability. Any Palestinians involved may be trying to get the spotlight on their cause before further releases.

``If the American professors are being held by Palestinians tied to the PLO,'' says one expert, ``they better get rid of them fast before they are found out,'' or risk serious public relations damage to the PLO.