Last February, in a lettuce field near this desert town just above the Mexican border, a striking farmworker named Rufino Contreras trespassed and was shot to death.

The killing came at a critical time in the Imperial Valley lettuce strike. It drew national attention and it sparked demands for justice from Cesar Chavez' United Farm Workers. But 2 1/2 months later, when the strike and the attention had moved elsewhere, murder and assault charged against three men accused of the killing were unexpectedly dismissed.

At no time, however, has the case been dismissed in the minds of the Contreras family -- Rufino's widow Rosa, his brother Luis, his father Lorenzo. Now their concern is shared by others who have examined a recently prepared transcript of the preliminary hearing in El Centro Municipal Court, where the dismissal occurred.

The transcript has deepened the suspicions of those who thought that the case merited a trial by jury. Among other things, the transcript revealed that the prosecutor, Imperial County District Attorney Fielding Kimball, had not even attempted to make a closing argument urging that the defendants be tried.

Judge William E. Lehnhardt, who once defended his dismissal ruling on strictly legal grounds, now acknowledges that Kimball's attitude helped influence the outcome. Asked whether the lack of argument was significant, the judge replied: "That helped -- the D.A. didn't care."

Interviewed here in his small, air-conditioned chambers on a 107-degree day, the judge said that Kimball's lack of enthusiasm for further prosecution had to be taken into consideration.

"He [Kimball] didn't actually say, publicly, in front of the defense lawyers, that he wanted the charges dismissed, but he let it be known," Lehnhardt said. "He wasn't ready to take that stance publicly. It had to be me." The judge would not specifically confirm or deny that he and the prosecutor reached their understanding in chambers, but he made it known that on the crucial point of dismissal, communication between them was unmistakably clear.

El Centro, the hub of a fertile, agricultural valley where most of the nation;s winter iceberg lettuce is grown, was torn with labor strife at the time of the Contreras shooting. Growers claimed that their non-union workers were targets of UFW intimidation. Using mass picketing techniques and sometimes driving workers from fields, the union brought production at several of Imperial County's largest lettuce operations to a near standstill.

The conflict was at its height on Feb. 10 when Contreras, a 27-year-old Mexican farm worker described by those who knew him as quiet and gentle, entered a lettuce field owned by Mario Saikhon, considered by the UFW to be among the most anti-union of the growers.

Most of the fields where crews were harvesting at the time were guarded by Imperial County sheriff's officers. But on Feb. 10, most of Sheriff Oren R. Fox's deputies were 20 miles away, trying to head off a violent confrontation at another field. Fox says the growers had assured him on the night of Feb. 9 that there would be no work in the Saikhon field the next day.

On the day of the shooting, 40 pickets, some of them armed with sticks and clubs and shouting cries of "scab, scab," in Spanish and English, crossed into the Saikhon fields with the evident purpose of chasing off three harvest crews comprised mostly of Filipino and Anglo workers. Entering from the southwest corner of the field, they ran north along the lettuce rows. Contreras and five others were in front of the main body of pickets, Shots rang out. Contreras took a bullet in the head, just beneath his right eye. He fell face downward, his body sprawled across three rows of lettuce.

After the shooting, angry strikers clustered around the edge of the field, demanding the arrest of three men they said had fired at them from separate locations. Fox and his men rushed to the scene. By the end of the day the sheriff's department had arrested Saikhon foreman Anthony San Diego, 29, Leonardo Barriga, 53, and truck driver Froilan Mendoza, 39. Two days earlier, in another field, deputies had confiscated a pistol which San Deigo had tucked into his waistband.

San Diego, who came to the Imperial Valley soon after the strike started, was on probation for assault with a deadly weapon in another county. In 1977, he was convicted of striking a man in the mouth with the handle of a knife and then, while holding the knife to the victim's throat, threatening to kill him.

UFW strikers, while encouraged by the speed of the sheriff's response, were concerned that political realities might make it difficult for a local district attorney to actively prosecute the killing. The union urged that a special prosecutor be assigned to the case by the state attorney general, but Kimball argued successfully that he was fully capable of prosecuting the defendants.

Kimball's most obvious legal difficulty was that the bullet which killed Contreras was never found -- despite a hands-and-knees search by sheriff's deputies and National Guardsmen equipped with metal detectors.But the lesser charge of assault with a deadly weapon was not so problematical. Several witnesses, in sworn statements, declared that they had seen the defendants firing at them in the fields, where even possession of firearms would have violated an ongoing court order. The sheriff recovered at least three pistols believed to have been used in the incident, including a .357-magnum revolver taken from a member of the harvest crew and later stored in Saikhon's safe.

In California courts, the first stage of a criminal proceeding is usually a preliminary hearing at which a judge decides whether a case should go to trial.

The preliminary hearing in the Contreras killing opened on April 2, continued for a week and was delayed in mid-proceeding for two weeks. It was resumed on April 16. By the time Lehnhardt granted a defense motion and dismissed the charges on April 23, the focus of the lettuce strike had shifted to the Salinas Valley, more than 500 miles to the northwest. The UFW was outraged by the judge's decision, but the union was preoccupied with the strike, and there was little it could do to bring pressure in faraway El Centro.

The dismissal came as a surprise to many who had followed the case, including members of the southern California press, which had given close coverage to the strike and the killing of Contreras. However, no reporters were in court the day charges were dismissed, apparently because they assumed the defendants would be held for trial.

In winning dismissal of all charges, the three defense lawyers never called any of the defendants to the stand. They based their case on a self-defense argument, even though they refused to concede their clients had done any of the shooting. Relying on testimony from crew members who had been working in the fields, the defense attorneys argued that some of the firing had come from an unidentified rifleman, said to have been standing in the corner of the field where the strikers entered.

UFW members say there uas no union rifleman and that he is an invention of the defense. They point out, accurately, that the growers never even hinted at the existence of a union firearm in the press statements they issued after the shooting. At the time, the growers said only that the strikers carried clubs.

Though there have been a number of allegations of union violence during the strike, all incidents are said to have involved rock-throwing or the use of pipes and clubs. Neither Judge Lehnhardt nor Public Defender Roger Cognata recalls any incident involving firearms in the possession of strikers. Fox said a thorough search turned up no evidence of the supposed union rifle, of anyone who might have fired it, or shells in the area where the rifleman was said to have been standing.

In fact, the only rifle found by a sheriff's deputy after the shooting was in the pickup truck of a non-union camp cook. Kimball called the cook as a witness, but his testimony was ruled inadmissible because no copy of a statement he gave to a deputy was provided to defense counsel.

The testimony concerning what union workers say was a phantom rifle came at the preliminary hearing from four crew members, all Anglos. One of them previously had told a sheriff's investigator he saw a rifle, but under oath would say only that he thought at the time he had come under fire. Another testified he heard shooting but saw no rifle or firearms of any kind in union hands. A third said he was positive he saw a rifle, but did not see anyone fire it. The fourth said he saw a rifle, but then acknowledged under cross-examination that he may have been mistaken.

One crew members, who said he never saw any weapons at all, assumed the shooting was directed at the strikers. He told sheriff's deputies, "many strikers hit the dirt when the shots were fired." But he was never called to testify.

The supposed shooting from the strikers' side, Lehnhardt said, was a leading factor in his decision to dismiss the charges. The testimony, the judge said, "leaves the court in a situation where perhaps there is a suspicion of guilt, but it simply doesn't raise to the level of being a strong suspicion of guilt."

In California courts, such questions usually are left for a jury to decide. But the Contreras case was not a usual one. At least three attorneys said that Kimball had gone to Lehnhardt and told him that he wouldn't object if the case were dismissed. Lehnhardt, however, said that such a straightforward step wasn't necessary.

"You can get across these messages without saying so in so many words," the judge says, in looking back on what happened.

Asked again whether Kimball had specifically requested dismissal, the judge said: "He did indicate he wasn't going to argue vigorously. Let's put it that way."

It is now Kimball's recollection that he "argued vehemently" against dismissal even though the transcript shows that the 63-year-old prosecutor made no argument at all on the renewed motion for dismissal made after the defense rested. Five days earlier, when he had closed his own argument, Kimball opposed dismissal primarily on the grounds that the defense had not yet introduced evidence to support a self-defense.

"So I would submit at least we should hear some affirmative testimony," Kimball said then.

After the defense presented its witnesses and renewed its motion to dismiss, Lehnhardt asked Kimball for the prosecution's position. This was what the district attorney said:

"The court heard my previous remarks when the other motion was made and the court has been attentive. And I noticed the court has taken, conscientiously taken, notes. You have heard the evidence. I'm going to submit it on the basis of the what the court has heard."

Kimball says now he was concerned mostly with the murder charge because "assault isn't a serious enough charge" to continue prosecution. The district attorney observed that he could refile the case at any time, but added: "I have no reasonable anticipation of doing so."

In evaluating the curious outcome of the proceeding, Kimball defends his own performance and blames the UFW for failing to produce needed witnesses.

The UFW insists it cooperated with the prosecution, and the union has been trying ever since the dismissal to have the charges reinstated. In any case, the record is clear that Kimball did not bother to call as witnesses some identified strikers who were eyewitnesses to the shooting. One of these, Arthur Mendez Fedrico, told a sheriff's deputy that he saw all three defendants firing and added that "while we were standing over Rufino's body, San Diego and Mendoza started shooting at us again."

Since the bloody farm strikes of the 1930s, union organizers in Imperial County have been suspicious of a local government they regard as grower-dominated. Law enforcement here has been viewed similarly by more militant elements of the Mexican community. Local law officers, they say, are always ready to break a strike or put down a demonstration.

This attitude has softened in recent years. Sheriff Fox, who has a reputation for even-handedness, was elected in 1978 with significant Mexican-American support.

Even today, however, Public Defender Cognata says that no UFW member arraigned in court against local farming interests has any hope of receiving a fair trial.

"This is a small county," he said. "Everybody's relating to the farming interests in one way or the other."

Cognata is so convinced of the difficulty of obtaining a fair trial here that he automatically requests a change of venue for all Mexican farm worker clients charged with strike-related offenses.

This year some of the old bitterness between Anglos and Mexicans was rekindled by the early success of the lettuce strike, now in its ninth month. In the past, growers had little trouble breaking Imperial County farm strikes because of the reservoir of available Mexican labor just across the border. This year, however, the overwhelming majority of Mexican workers, whether UFW members or not, heeded Cesar Chavez' plea and refused to help growers break the strike.

This new-found Mexican solidarity symbolizes the growing economic and political strength of the farm workers that some hope will be brought to bear in the Contreras case and lead to a reopening of the investigation into the farm worker's death.

"While people are thinking it's ended [the Contreras case] I don't think it has," says Carlos Alcala, a Harvard-trained UFW lawyer from Salinas.

Alcala has filed a complaint with the California Agricultural Labor Relations Board, alleging "that in killing Rufino Contreras, the Saikhon Corp. violated the rights of Rufino and the others to organize." Alcala anticipates that the board will take up the matter, issue subpoenas and hold a hearing. A board spokesman would say only that the case is under investigation.

While the U.S. Justice Department has maintained that it lacks jurisdiction, the FBI recently completed a confidential preliminary inquiry into the shooting that was turned over to the department's civil rights division. Last Thursday, U.S. Sen. Harrison Williams (D-N.J.), who as chairman of the Senate Labor and Human Resources Committee heard testimony on the Contreras shooting during a Salinas hearing last April, asked the Justice Department to review the seven-volume transcript of the preliminary hearing.

Williams' request came in a letter to Attorney General Benjamin Civiletti, who promised in a recent Washington speech to the National Council of La Raza Unida, a Mexican-American political group, that the Justice Department no longer will ignore allegations of civil rights violations just because state or local agencies are investigating.

Meanwhile, Rosa Contreras, Rufino's widow, waits for word in the Mexican border town of Mexicali that the case will be reopened. While it has been eight months since her husband's killing and she has only vague assurances that there will be a new investigation to sustain her hopes, she believes her husband's killer will be brought to justice.

She talked to reporters in the tiny living room of the screenless house where she lives with her two children, Nancy, 5, and Julio, 6, in an unpaved, working-class section of Mexicali. Though the temperature was well over 100, she wore a pants suit to conceal severe burns on her legs suffered in a gasoline fire shortly before her husband was killed. Her living room contains a couch, a table, a chair, a television set, and an inexpensive stereo. The room is dominated by a picture of Rufino, smiling sadly from behind a cracked glass frame.

He was, she says, "muy pacifico," a quiet man who never picked fights with anyone. It is a view shared by her husband's fellow workers and by her brother-in-law Luis, who lives next door and was with Rufino in the field when he fell. His brother, said Luis, was not a militant person, but peaceful and good-natured.

"No bravo," said Luis, shaking his head. "If you spoke to him, ;e spoke to you."

Rosa is a quiet person, too. She lives these days on Social Security checks, small payments from the union and the hope that the prosecution will be resumed.

Looking at the picture of her husband, she said in Spanish: "I believe justice will come in the end. If it doesn't, there's nothing in the United States to believe in."