When the decomposed body of a middle-aged woman was found floating in the Lujan River here Jan. 11, nothing seemed particularly unusual at first. Thirty-five bodies had washed up on a beach not far from Buenos Aries in December, their heads and hands severed to prevent identification.

But the woman's body was intact and identification was possible. It is unlikely that the killers will ever be prosecuted, even if they are caught, but the fact that Elena Holmberg -- a diplomat and friend of President Jorge Videla -- ended her 48 years as a victim of terrorism has shocked Argentina as have few other terrorist acts in recent years.

"Her death was important simply because it won't go away," said a European diplomat who watches the human rights situation here. "The emotional reaction to it was greater than to anything else that's happened in a long while."

The case has had such impact because a growing number of Argentines have reluctantly come to the same conclusion as many foreign observers: Argentina is threatened more seriously by its own internal security forces than by the urban guerrilla groups they were meant to eradicate.

These security forces, which almost all who comment blame for the Holmberg death, also are assumed to be responsible for the extralegal abductions of thousands of suspected subversives here since 1976. It was then that the military overthrew President Isabel Peron and the "dirty war" against terrorism entered its current phase.

The bodies that wash up on beaches are evidence of what happens to those who the secret squads believe have engaged in terrorist acts, according to diplomatic observers and human rights groups.

But now that the leftist guerrillas have almost been defeated, the secret intelligence squads -- equipped with guns, unmarked cars and methods for interrogation and torture -- have turned to petty crime, to picking up non-terrorist leftists or, as in the case of Holmberg, to silencing even government supporters who might somehow embarrass one of the three armed services.

"Argentina has become like Chicago in the 1930s," said another diplomatic observer. "Each service has its own hit squads which operate freely beyond the law."

Holmberg's abduction was typical of those by the secret services and the government has made no effort to blame leftist guerrillas although it frequently does so. As a diplomat said, "The Montoneros just don't have the ability anymore to carry out an operation like this on a busy street in downtown Buenos Aires, with witnesses all over the place.The regular police would have stopped it, or tried to have stopped it, had the intelligence services not had clearance beforehand."

It is unlikely that anyone will ever know precisely why Holmberg was killed or what she might have known that was potentially damaging. But "the general theory is that she knew too much and talked too much," the European diplomat said.

Had Holmberg been associated with leftist causes, her death probably would not have been reported here -- much less have caused the public reaction that continues almost two months after her body was found.

But Holmberg was a diplomat who until six months ago was stationed in Argentina's embassy in Paris and actively involved in the military government's public relations campaign aimed at countering its image as a gross violator of human rights.

Many diplomatic observers and informed Argentines believe Holmberg learned something in Paris about one of the military services that was the cause of her abduction when she returned to Buenos Aires.

Since she was known to have close ties with the army, most observers think she must have been done in by the navy. But the possibility that any of the three armed servies could have been responsible illustrates "that really the whole thing is out of control," a diplomat said.

Ironically, Holmberg fervently believed in the methods used by the intelligence squads, the methods secret intelligence squads, the methods of secret abduction, torture and execution that have caused Argentina's image problems and that apparently were used on her

She often told friends that she believed these internal security squads might have to eradicate 100,000 more "subversives" if Argentina was to overcome its chronic political instability.

"She was very outspoken on this," a man who knew her well said the other day. "She believed that many people should be killed."

Although Holmberg was not rich, she was a member of one of Argentina's most socially prominent families. Her father, Adolfo Dago Holmberg, 90, is one of the country's most famous naturalists. On her mother's side, Elena Holmberg was related to former president, and general, Alejandro Lanusse.

Elena Holmberg was also a personal friend of Gen. Videla, who expressed "anguish and grief" shortly after she was abducted in broad daylight near her home in the center of Buenos Aires Dec. 20.

Foreign Minister Carlos Washington Pastor, a retired Air Force general and Holmberg's boss, said then that the government was doing everything it could to find her.

Although diplomatic observers have long believed that Videla has little control over the secret squads, his inability to prevent Holmberg's death was the first time that his lack of authority was demonstrated so publicly.

"Nobody's safe, that's what this murder has shown, especially to those on the right who supported the military and thought they were immune," a diplomat said. "It has shown that Videla is powerless to stop the military forces he should theoretically command... In a sense, the revolution has begun to devour its own children and nobody can do anything about it."

A prominent civilian, whose house was bombed several times during his service in the current government, probably by navy or air force security squads as a warning to Videla, said Holmberg's death "has scared people. They are afraid that if they get involved or express their opinions, then they will end up with a bullet in their head or in the river."

The reasons that the squads have not been brought under control, according to several diplomatic observers, are complicated. The three armed services are bitter rivals behind the facade of thee ruling junta. Each service refuses to do what another service might suggest.

Each service is also divided. Certain admirals and generals are more hardline than others and corps commanders often refuse to accede to the wishes of the central staff. Since the antiterrorist squads generally work at the corps level, a hard-line corps commander may allow them to operate while denying responsibility to his superior.

Since the squads originally were formed as self-contained units to avoid infiltration by the guerrillas, who were suspected of having sympathizers within the armed forces three years ago, some of the squads may answer to no one other than the captain or corporal in direct command.