Nothing was mysterious about Fermin Garcia Guardado's reticence to talk. The Salvadoran farmer was still in mourning and deeper in shock. At dawn on Feb. 22 in Las Hojas, a rural village in El Salvador, one of his eight children, a boy of 18, was routed from bed by uniformed soldiers. They led him away, to be thrown with a pack of other terrorized villagers. Later in the day, the inevitable gunshots were heard.

The son's corpse was among 18 bodies found outside of town. Before long, other murdered peasants--as many as 72--were discovered.

Several weeks after the killings, I interviewed Garcia Guardado. He had come to Washington, under the sponsorship of the Unitarian Universalist Service Committee of Boston, to tell congressmen about the massacre. The thin, wrinkled-faced farmer, who is the treasurer of the 15,000-member National Association of Indigenous Salvadorans, was an inspiration of fearlessness. He was using his real name. By talking about the killing of his son and fellow villagers, he was describing a real event: a massacre, not an "anti-subversive sweep," as the American-trained Salvadoran army calls it.

By being in Washington, Garcia Guardado knew he could be a marked man on return to his country.

That is how it turned out. In early July, news reports, first in The Washington Post and later The New York Times, told of the jailing of Garcia Guardado. The Salvadoran National Police charged him with shooting and wounding a villager who had helped the army on the day of the killings.

From the news stories, it can be fairly concluded that the charges are as empty of substance as are the efforts of the Salvadoran government to prosecute the military death squad that carried out the massacre. It gets worse. The government's public prosecutor, the defense minister and Alvara Magana, the Salvadoran president, all received a carefully documented report from El Salvador's Human Rights Commission that traced the massacre to one captain and his soldiers.

The defense minister publicly promised--this being a case where unavoidable facts were well-known--that a full investigation would occur. Justice would be done. Today, with Garcia Guardado in jail, the accused captain, called an "excellent" officer by superiors, remains free.

The killing of one 18-year-old peasant, or even the massacre of 72 peasants, is only a drop in the bucket of blood that is El Salvador. This event differs from the terror that has become routinized the last four years because it reveals that the United States is dealing with a military government with a weak civilian facade. It is not accountable to laws.

The findings of the Las Hojas massacre have never been in doubt. President Magana has the authority to act but not the power. The military would not carry out his orders. Instead of governmental laws, the country is ruled by military wishes. The army is dominated by local military commanders who operate like feudal chiefs. The elections of March 1982, when democracy was supposed to take hold, did little to control the military.

Last week, the Reagan administration, asking Congress for more military aid, obeyed a 1981 law that requires evidence every six months that the receivers of the aid have progressed in human rights. The law was obeyed but the truth was crushed.

This certification process has been a deception from the beginning, going back to the last months of the Carter administration when El Salvador's Archbishop Romero, soon to be slain, pleaded that no more military money be sent. It was, and the army, being an army, has been using it well. In the six months since the last certification, known civilian deaths have increased by 12 percent. This rise is based on figures from El Salvador's Catholic human-rights office: 437 monthly civilian deaths from January through May, against a monthly average of 390 killings for the previous half-year.

Instead of realizing that strengthening the Salvadoran military only increases its ruthlessness, which was Romero's unerring prediction, the Reagan administration is befogged by the numbers. A spokesman says the new figures on civilian deaths suggest "there's a slight increase, but we're not sure if this is a trend up or down because the numbers do fluctuate."

If the Reagan administration policymakers can't tell when up is up--do they think a 12 percent increase is a decline?--then why shouldn't the Salvadoran military feel free to let killers go free while jailing the families of its victims?

In Washington, up is down and in El Salvador carnage is freedom.