More than 40 government soldiers were killed one night last week among the crude bunkers of piled rocks and shallow trenches overlooking a strategic bridge at a bend in the Pan American Highway called Quebrada Seca. Three or fewer others were taken prisoner by the rebels. The smell of the dead still lingered there yesterday.

In the auditorium of the U.S. Embassy here today, 49 glossy 8x10 color photographs of corpses were shown to reporters as proof that 30 of the 42 dead soldiers counted by the government were the result of what U.S. military group commander Col. John D. Waghelstein called "an execution" of men who had surrendered.

"My gut feeling is that there is a change in guerrilla tactics," Waghelstein said. The first of several theories he expounded for this suggested that "the hard-liners have gotten control" of the rebel movement and "the gloves are off."

The presentation here on the May 25 killings followed distribution Tuesday by the State Department in Washington of a similar account, identifying the locale by the nearby town of San Francisco Chamoco. The department spokesman also said a possible explanation of the killings was a shift in guerrilla tactics.

For almost a year now the guerrillas, in marked contrast with the government forces, have taken prisoner those soldiers who surrendered. In most cases they have released them quickly, sometimes in a matter of hours.

The guerrillas credit the tactic with cutting down resistance to their attacks and letting them take many small government outposts after little more than a show of force. The government is vague about the number of soldiers who have given up this way, but the rebels claim more than 1,000, and their control over terrain clearly has grown as government commanders give up staffing many small garrisons.

In a broadcast over the rebel radio station, one of the top guerrilla commanders, Joaquin Villalobos, said that the rebel prisoner policy remains the same and pointed to the taking of more than 40 captives after an attack on a radio relay station this week as evidence.

Waghelstein today pointed out that one effect of Quebrada Seca is that it "certainly will toughen the spine" of units that might otherwise have given up.

But while evidence of some cold-blooded killing is strong in this case, many of the broader conclusions being drawn about it appear founded on the U.S. Embassy's speculation.

Waghelstein suggested that the murder of his second in command, Navy Lt. Cmdr. Albert A. Shaufelberger III, and reported guerrilla executions at the town of Cinquera earlier in May might all be evidence of the change in guerrilla tactics. He said that the incidents could represent "a message" to the American people. As an alternate theory he raised the possibility of a major break between the guerrilla factions. The last "option" he suggested might be a simple loss of command and control in a localized incident.

After questioning on specific points, Waghelstein concluded, "It may be a little early to lock onto any of the options."

On the hot hillsides around Quebrada Seca yesterday, soldiers now manning the positions above a new prefabricated temporary bridge--supplied by the United States to replace the one blown by the rebels--offered simpler explanations of what might have happened.

A 19-year-old private suggested that after a night of heavy fighting in which the guerrillas may have suffered extensive casualties, they were too angry to bother with prisoners.

Seventeen-year-old Oscar Armando Perez, in the first group of soldiers to arrive on the scene when the fighting had ended, said there were bodies everywhere--"26 died here, right here in these trenches." Perez gestured at the ground around him, where no one had picked up bloodstained bits of uniform. Asked how he knew that many of them had been executed, Perez said, "When you go into battle you fall twisted and the bullet that gets you doesn't get you only in the head."

Asked specifically where the bodies he found had been shot, the boyish soldier said, "Some here," and put a finger on a point between his eyes, "others here," and he touched the back of his neck, "and others had their head all taken apart with the brains coming out the back."

According to Col. Jose D. Hernandez, the brigade commander in the city of San Vicente whose troops were involved, there were 82 men guarding the bridge when it was attacked at about 10 p.m. May 24. Although he ordered about 150 men stationed three miles away to relieve the post, that column was caught in ambushes laid by what Hernandez estimated was a force of several hundred, perhaps more than a thousand, guerrillas.

A new relief column was not sent out from the San Vicente barracks until 5 a.m. the next day, Hernandez said, arriving on the scene at 6:30. By that time, the rebel forces were under fire from an A37 jet and had begun to fall back.

Hernandez said the rebel attack on Quebrada Seca represented the strongest offensive guerrilla action ever seen in San Vicente Province--"bigger, with more people, more terrorists, who were well-armed." Many of his soldiers appeared reluctant to talk about it, not so much because of the massacre as because it seemed a serious defeat.

Guerrilla commander Villalobos listed it among the victories of his and other rebel groups in the latest guerrilla offensive. "The U.S. Embassy comes and talks about human rights because in a combat there was a higher number of casualties than were produced in other opportunities," Villalobos said.

Key figures in getting out the publicity about the incident were a group of American advisers training the troops in San Vicente and working on planning and operations there. Three advisers were brought to the embassy press conference today to describe what they had seen and been told by soldiers.

The count of 30 dead soldiers with bullet wounds in their heads and nowhere else was made by these Special Forces officers and sergeants, one of them a medic. They said that most of the head wounds had the dark blotches around them left by a bullet fired at point-blank range.

The sergeant who took the 49 color photographs, processed in Panama, at first said he had photographed all 42 bodies. But fewer than 20 different corpses were actually counted in the pictures. Asked about this, the sergeant said, "Some of the bodies I didn't take the pictures of because they were already bagged and being prepared for burial and they were nude."

There was little publicity about Quebrada Seca in the Salvadoran press when it happened. Salvadoran officials "botched a golden opportunity to take advantage of the propaganda value" in the event, said one military observer. The Embassy has worked to pick up that slack. The first major reports of the killing to appear in El Salvador's newspapers were wire service stories from Washington, where the State Department spokesman gave his account Tuesday based on an embassy cable declassified and distributed there.

On the day the incident occurred, an embassy official had dropped by the hotel where most journalists stay to tell several of them the story and encourage them to publish it. But Shaufelberger's murder that night overshadowed everything else until the beginning of this week.