The murder of four American missionaries in El Salvador last December is one that could be solved by your ordinary late-night television movie detective before the first commercial.

The four women arrived at an airport, traveled along a road that was patrolled by the national guard, were slain and hastily buried. Kojak or Barnaby Jones would have simply called for the duty roster, commandeered weapons and with a minimum of questions, found the culprits.

But for some reason, the El Salvador junta cannot crack the case. Six men were arrested last April and are still detained, but no charges have been brought. The Reagan State Department, which takes umbrage so quickly in other matters, has slandered the victims and stonewalled their inquiring relatives. No new information has been found, and the investigation, such as it was, has come to a dead stop.

But outside the administration, in Congress and among ordinary citizens who smell a Vietnam-type involvement, the ghosts of the four women walk--and thwart plans for further military aid.

And no one is more grateful than the beleaguered president of the junta, Jose Napoleon Duarte, who wound up a recent 10-day stay here by meeting with the relatives of the four women. He must thank God every night on his knees for the one weapon he has against the murderous military whom he unconvincingly professes to control. He has to be unspeakably be- holden to the brothers and sisters and mothers and fathers who have persisted in what the State Department plainly views as an unseemly, and unpatriotic, quest for justice.

Duarte's visit here seems to have been one long cry for help. He has been under constant surveillance. The chief of the internal security forces, a Colonel Casanova, accompanies him everywhere--and, of course, is therefore a witness to the swarming questions about the nuns.

When he went before the Senate Foreign Relations Committee last week, Duarte enthusiastically acceded to conditions to be placed on the military aid that the Reagan administration is so anxious to confer. The next day, however, in a letter, he took back his endorsement. Liberal senators suspect that Col. Casanova stood over him as he wrote.

Duarte's answers about the unsolved murder are weak, and he knows it. The El Salvador law he feebly cites seems to be made up for the case, as it goes along. At the meeting with the families, which was held at the residence of Archbishop James Hickey, the acting minister of justice was present, but she was unable to enlighten anyone even on the basic matter of whether the soldiers would be tried in a military or a civilian court.

The meeting was arranged through the good offices of Rep. Mary Rose Oakar (D-Ohio). The archbishop, friend and onetime pastor of two of the victims, has been, to the dismay of his fellow Catholic, the secretary of state, a leader in resistance to a military solution in El Salvador.

The families held a press conference after their meeting with Duarte, and from it emerged a picture of a desperate man playing the only card he holds.

Duarte told the group that on his first meeting with President Reagan, he had asked for help in solving the crime, FBI assistance, lie detectors.

He had no information to give the families. Plainly, he was just hoping to signal the military that they are in trouble with Uncle Sam.

The current U.S ambassador to El Salvador, Dean Hinton, was present, and it was pretty clear whose side he is on. He spoke of the danger of "setting a precedent by having the FBI investigate a murder outside the country."

He and Bill Ford, the brother of Sister Ita Ford, had a spirited exchange when Hinton said the investigation should be limited to the six men "who were acting on their own." Ford said he was "offended" at Hinton's vindication of the military.

The families asked Duarte to ask Reagan for a high-level State Department-FBI team to go to El Salvador and get to the bottom of the thing.

Dorothy Kazel, the blond sister-in-law of Sister Dorothy Kazel, had asked Duarte what wrong the missionaries had done "that the military would kill them in cold blood?"

Duarte said that these women were "martyrs" who wanted only to save the people of El Salvador. "As far as he was concerned," the families reported, "they would be called saints."

Having heard their loved ones described by American officials as "political activists" and gun-toting roadblock runners, the relatives were gratified to see Duarte observing the decencies which evaded the secretary of state and the ambassador to the U.N.

But Jean Donovan's frail mother said hardily that she wanted to "wait to decide" as to just how sincere Duarte is.

The group put to him the obvious question. Does he dare bring the killers to book? He told them he was not afraid of losing his post and that his life is in danger anyway.

Bill Ford succinctly outlined the politics of Duarte's position:

"I think it was clear to everybody there that Duarte did not realize until this trip how serious this issue was in the eyes of the American people."

Ford added tartly, "It was clear he got no sense of that from the State Department."

The families are perfectly willing to lend a hand to Duarte, to whom the dead women have thrown out the lifeline. He is in the hands of thugs, the unsolved murder perfectly makes the point. He needs detectives, not grenade-launchers, and if he can convince the Reagan administration of it, his trip here will have been a dangerous success.