PANAMA CITY -- Riding in a cream-colored Chevrolet Impala with Michigan license plates, Marine 1st. Lt. Robert Paz and three fellow officers were rather conspicuous as they drove past Gen. Manuel Antonio Noriega's downtown military headquarters on a Saturday night last month.

Although they wore civilian clothes, their car and their short haircuts readily identified them as members of the U.S. military's Panama-based Southern Command, a main target of a two-year anti-U.S. harassment campaign by Noriega's forces.

The driver of the car, another Marine lieutenant, had been in the country for three months and did not know his way around very well.

Their outing on Dec. 16 ended in a shooting incident that left Paz dead and unleashed a chain of events that is still unfolding: U.S. military intervention in Panama, hundreds of casualties on both sides and the overthrow of Gen. Manuel Antonio Noriega.

The U.S account said the marines fell into hostile Panamanian hands by chance. Circumstances surrounding the incident suggest that contrary to some initial suspicions -- raised by the Noriega government's version of the incident -- the officers were not on a hostile mission when they drove by Noriega's headquarters, known as the Comandancia.

As reconstructed from several sources, the fateful events began when the four officers left the back gate of Fort Clayton at about 8:30 p.m. on their way to have dinner at a Panama City restaurant. In keeping with a curfew set by the U.S. military for the 12,000 members of the Southern Command, the officers had to be back at their base by 11 p.m.

Although some questions about the incident persist, such as why the officers left their base so relatively late, no evidence has emerged in the two weeks since the shooting that substantially contradicts the U.S. version of it. The Noriega's government's account -- that the officers opened fire at the Comandancia from their car and wounded three passersby -- proved to be a fabrication.

According to the Southern Command, the whole incident stemmed from a wrong turn that led the four Marine officers down a maze of one-way streets and brought them in front of the Comandancia, but that -- given the anti-American indoctrination and psychological pressure being exerted by Noriega on his own forces -- it was an accident waiting to happen.

The day before the incident, a legislature appointed by Noriega had named him head of government and declared Panama "in a state of war" with the United States. Speeches before the assembly were filled with anti-American rhetoric, and at least two speakers called for the killing of "gringos."

But according to U.S. officials, another factor may have made the troops and paramilitary units guarding the Comandancia jumpy.

"Noriega was fearful he was going to have a coup on Dec. 15," said one U.S. military officer with access to intelligence reports. On Oct. 3, Noriega's supporters had used unprecedented brutality to crush the second coup attempt against him in 19 months, adding to a climate of fear, suspicion and vengeance within the 16,000-member Panama Defense Forces (PDF).

When Lt. Paz, the 25-year-old son of missionaries in Colombia, and the three other Marine officers drove into the rundown Chorrillo quarter where the Comandancia is located, it was shortly after 9 p.m.

By the U.S. account, as they approached the PDF headquarters on Avenue A, the officers were stopped at a checkpoint by about five uniformed members of the PDF's Battalion 2000 and a crowd of up to 40 civilians, some of them believed to be members of Noriega's paramilitary Dignity Battalions. The PDF soldiers tried to pull the servicemen out of the car, but could not because they were wearing seat belts.

The driver of the car, who spoke to a couple of reporters briefly five days after the incident but declined to give his name, said he noticed that a PDF soldier at the checkpoint had readied AK-47 automatic rifle and was poised to fire. By then, two cars ahead of the Americans at the roadblock had driven through, leaving an opening.

"So I stepped on it," the driver said.

The five PDF troops opened fire from the rear as the car sped away, hitting Paz, who was in the back seat. As the car passed the Comandancia on the left, guards inside it, believed to be members of Noriega's staunchly loyal Machos de Monte counterinsurgency company, also opened fire. The driver said the car was hit by at least eight shots, one of which grazed him in the left ankle.

As one of the officers in the car tried to revive Paz, the driver made another wrong turn as he reached 4th of July Avenue. The mistake cost several more minutes in reaching the Southern Command's Gorgas Army Hospital. Paz died about 15 minutes after arriving.

The driver and other American officials said the four officers were off-duty and unarmed.

In a statement issued seven hours after the incident, the PDF claimed that the Americans had fired at the Comandancia from their car, wounding three Panamanians including a year-old girl. However, the girl's grandmother said at the hospital that she actually had been hit by a stray bullet in an unrelated incident four miles away, and it appeared likely that the other two were wounded in the PDF's wild firing at the escaping Americans.

The shooting was witnessed by a Navy lieutenant and his wife, who also had mistakenly driven up Avenue A while returning to Rodman Naval Station from a popular Panama City restaurant. Officials said the two had been stopped at the roadblock about half an hour before the shooting and had been told to pull over and wait in their car while their identities were checked.

By this account, when the PDF realized the couple had seen the shooting, they were handcuffed, gagged, blindfolded with masking tape and taken for interrogation to a location believed to be the PDF's notorious G2 military intelligence headquarters.

The Navy lieutenant, who a military source said held a "sensitive job," reportedly was kicked in the groin, punched in the head and threatened with death as interrogators questioned him about his unit's activities.

According to an officer who debriefed the lieutenant, "a loaded weapon was placed to his head and ears" during the PDF interrogation, and he was "lectured on how Nicaraguan, Salvadoran, Cuban and M-19 {a Colombian rebel group} brothers would join the regime to kill Americans."

The lieutenant's wife, a university student of international relations who had arrived in Panama that afternoon for a visit, sustained a cut on her head when she was slammed against a wall at one point during a separate interrogation.

All the while, a debriefing officer reported, the PDF interrogators made "vulgar and sexually degrading threats" against her. She later passed out after being made to stand against the wall with her hands over her head, the report said.

According to one Southern Command official, the PDF suspected that the woman was a CIA agent because she spoke Spanish and seemed knowledgeable about Panama, although she said she had arrived in the country for the first time only that day.

After about four hours, the two were released. They arrived at Rodman's Naval Security Office after 2 a.m. Sunday and gave statements about their ordeal.

The Southern Command declined to reveal the names of the couple or of Paz's three companions and refused requests to interview them in depth about the incident.

According to a well-informed civilian official of the Southern Command, the PDF probably had not planned to attack Americans that night, but their appearance before the Comandancia presented "a target of opportunity for gringo-bashing."

Given the tense climate and Noriega's anti-American "brainwashing sessions," the official said, "You could see that members of the PDF were getting psychologically loaded. Sooner or later something like this was bound to happen."

From their point of view, he added, the PDF soldiers were "protecting their Comandancia," and the Americans had run a roadblock.

From the U.S. standpoint, the soldiers had never asked the servicemen for identification as is customary at a checkpoint, had been aggressive and threatening from the start and had provoked the Americans' panicky attempt to flee.