Sometime in the early morning hours of Wednesday, the small organized resistance to the American invasion force advancing on this capital simply melted away, leaving an undefended city and a lingering mystery about the whereabouts of the revolutionary junta that provoked the U.S. landing.

I awoke at the St. James Hotel expecting to find the machine-gun nest and the dozen smartly dressed, coolly confident Grenadan soldiers who had occupied the street Tuesday night still there.

Instead, I found a group of civilians in swimming suits and shorts on their way down to gawk at the newly established U.S. Marine positions near the beach.

In the casually dressed group were three of the soldiers I had seen the night before. Now they were joking among themselves about their rapid change of clothes.

Within a few hours, heavily armed marines were moving down the street, crouching and bursting through doorways as if expecting house-to-house fighting. A French photographer who had passed through the same street earlier in the morning followed them, ignoring their repeated warnings to stay down.

The marines were passing through an information warp that extended not only all across this 133-square-mile island but all the way to Washington, where erroneous and out-of-sync official reports on the scale and intensity of the fighting in Grenada created conflicts and confusion that still have not been sorted out.

The Marines' exemplary caution in moving through an undefended city and the apparently misleading official statements in Washington appear to have flowed in part from the administration's reaction to unexpectedly strong resistance from Grenadan and Cuban combatants near the Cuban-built airport at Point Salines south of the capital.

There, the going was tough, and, according to Marine officers, the reporting from the 82nd Airborne Division's commanders at Point Salines tended to eclipse that from Marine commanders at Pearls Airport across the island and at Queen's Park north of St. George's.

Resistance at those spots was virtually nonexistent.

While the intensity of the ground war was being magnified in statements in Washington and in Marine planning, a wide-ranging and vigorous air war carried out by warplanes and AE130 gunships--the rapid-fire "Puff the Magic Dragon" of Vietnam--attempted to knock out antiaircraft positions.

It went virtually unreported in the statements coming out of Washington.

In other important aspects, the American invasion of Grenada looked different on the ground from how it was being portrayed from the official reports here and in Washington.

The gap resulted in part from efforts by the Reagan administration to filter all information through official channels by restricting access to the little Caribbean island, in a manner similar to that used by British authorities in managing the news during the war in the Falkland Islands last year.

The gap grew as reporters sought to fill the information vacuum with dispatches based on fragmentary information relayed anonymously by U.S. and Barbadian officials, and as the U.S. task force denied reporters access to Grenada and then refused to send dispatches from those few of us who had reached the island on our own steam in the early hours of the invasion Tuesday.

Thus, as Secretary of Defense Caspar W. Weinberger and Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, were announcing in Washington Wednesday afternoon that Richmond Hill was one of the two major pockets of resistance, U.S. marines already had occupied the area after meeting no opposition. Explosions shook the hill because of an exploding ammunition dump and unanswered outgoing U.S. fire.

It was as if American officials were believing the desperate bravado that the troops I encountered on Tuesday night were mouthing in what would turn out to be their last moments in uniform in St. George's.

They did not display the signs of panic or hotheadedness that I had witnessed frequently in covering bombardments in the Middle East and other Third World countries. They assured me that Gen. Hudson Austin, the Army commander, was "very much in charge" and that the members of the junta that seized power Oct. 14 "are in battle" at the front.

There have been no confirmed reports since the United States took the island of the whereabouts of Austin and his associates who precipitated the crisis by deposing prime minister Maurice Bishop. Bishop was executed on Oct. 19.

Nor is it clear what happened to the island's 1,200-man Army. Official American reporting has suggested that virtually all of the fighting has been between U.S. Army troops and the Cubans.

It is unclear if any number of the Grenadan military have joined the Cuban stragglers the United States says have taken to the hills before the advancing American units.

One young Grenadan I saw Tuesday night clearly did not. He was relaxed on Wednesday morning as he offered to take me down to the seaside to see the marines who had landed near the capital earlier in the day.

They had taken Pearls Airport easily the previous day, boarded ships and come around the island during the night, marine officers said.

As we walked, Grenadans weregingerly coming out of their houses onto the streets that they, too, thought would be occupied by the People's Revolutionary Army. Gradually they also began to head down toward Queen's Park. First arrivals were chased away by 10 marines lying in front of a bridge behind a machine gun. But the atmosphere eased as the morning went on.

By Thursday morning, young men were again playing soccer on the field that the marines had used as a landing zone for helicopters at Queen's Park.