JOHANNESBURG, SEPT. 21 -- The slaughter of commuters aboard a rush-hour train between Johannesburg and Soweto a week ago bore the stamp of careful planning and precise execution, factors that support a view here that there is an organizing hand behind the violence in South Africa's black townships.

Six attackers, their weapons concealed, boarded train No. 9436 at Johannesburg's Jeppe station. According to witnesses, they made no move until the train had made its first stop at George Goch station HED

two minutes later, after which, as a partial express, it passed through two other stations before its next stop at Benrose.

That gave the assailants 10 minutes to undertake their gory attack. They ran through coaches, shooting and hacking at passengers and hurling dozens through exit doors to fall in a tangle of broken limbs down steep embankments.

At Benrose station, another gang was waiting on the platform. When the doors opened and the panic-stricken passengers spilled out, this second gang attacked, shooting and slashing at more people with long machetes. In all, 26 people were killed and more than 100 injured.

None of the attackers said a word throughout the carnage, leading to speculation that they were not South Africans.

It was almost certainly this incident that convinced President Frederik W. de Klerk of the possibility that a "third force" was exacerbating the violence ravaging the Johannesburg-area townships in an attempt to destabilize negotiations between his government and the African National Congress. Since last month, more than 750 people have died in the fighting.

For weeks, de Klerk and his police minister, Adriaan Vlok, had rejected claims by the ANC, church leaders and civil rights movements, based on statements by scores of witnesses, that whites with blackened faces or wearing ski masks have been among bands of Zulu fighters rampaging through residential zones and squatter camps in the townships, and that the police have often stood by watching without intervening.

For example, witnesses said they saw white men wearing ski masks driving vehicles involved in two separate incidents in Soweto Sept. 6. In the first, black men jumped out of a minibus and opened fire at random on pedestrians, killing six blacks and injuring 30. In the second, four black men emerged from a pickup truck at a train station and opened fire on black commuters, killing five and wounding 14. Two days later, witnesses to an attack on a squatter camp in Soweto said whites with their faces blackened with soot were among the estimated 100 attackers who killed 13 people with guns and machetes.

De Klerk and Vlok have insisted that the violence was entirely due to political rivalry between supporters of Zulu Chief Mangosuthu Buthelezi's Inkatha Freedom Party and the ANC, which they equated with tribal animosity between the Zulu and Xhosa tribes.

But now the evidence of the train attack, and of several similar hit-and-vanish raids carried out with the precision of trained commando units, appears to have convinced de Klerk that some "hidden hand" also is involved.

He told a rally of his ruling National Party last Saturday that there appeared to be an unknown "third force," a "sinister conspiracy by well-trained terrorists or gangs" who were "using or misusing" the township violence.

On Wednesday, de Klerk ordered "special investigative units" from the Justice Ministry to examine charges of police misconduct in the township violence, and demanded that the train attack be investigated immediately.

That same day, police confirmed that at least one white man was riding with blacks in a van from which shots were fired randomly in downtown Johannesburg Sept. 12. Three pedestrians were killed, including a Portuguese woman.

In his speech last Saturday, de Klerk did not say who he suspects is involved in the "third force." "Who they are must be ascertained," he said. "It is not impossible that they come from within the ranks of the fighting factions themselves. Alternatively they may come from sources as yet unknown to us."

Black leaders, however, have been less hesitant about naming their suspects.

Mandela has stated on several occasions that elements within the security forces are fanning the township violence to destabilize his black nationalist organization's negotiations with the government. "It is quite clear that Mr. de Klerk is facing problems," he said in a recent interview. "De Klerk's inability to take decisive steps to stop this violence arises from the fact that some of these highly skilled, professional death squads are linked with the security forces in the country."

Sources point out that the only people who stand to gain from a destabilization campaign are the white extremists of Andries P. Treurnicht's Conservative Party and other elements further to the right. They regard the negotiations with the ANC on a new political system for South Africa as a sellout of the Afrikaner people -- descendants of Dutch, German and French settlers -- by de Klerk, and want to see the talks collapse.

These far-rightists say they have strong support from within the police force and other elements of the military-security establishment. Some white extremist leaders have claimed that 70 percent of the police are on their side.

Some sources, including several newspapers, have speculated on possible involvement by the Civil Cooperation Bureau (CCB), a special services branch of the Defense Force to which civilians and police personnel also belong.

On Sunday, the Johannesburg Star editorialized that "the belief grows that a 'third force' is stirring things. The specter of the CCB -- of men trained for destabilization, destruction and political murder -- looms. . . . The security forces have forfeited the confidence and respect of the townships. In residents' minds, they have been tried and found guilty of incitement, murder and brutality."

Today, the Daily Mail reported that elite units of Inkatha fighters were trained alongside Mozambican Renamo rebels at two camps operated by Defense Force officers as part of a destabilization operation. The report was based on sworn affidavits of former members of the Kwazulu police force. The insurgent Mozambique National Resistance, known as Renamo, has carried out what it describes as an anti-Marxist campaign against the Mozambique government. In the past, Renamo reportedly relied on military aid from South Africa.

According to the Daily Mail report, one of the camps was located in the Kwazulu homeland, whose chief minister is Buthelezi. Kwazulu is located within Natal province, where Inkatha is based and where years of fighting between Inkatha and the ANC have taken thousands of lives. The report said the camps were under the control of the chief of staff for military intelligence, but the Defense Forces denied knowledge of their existence.

The security underworld of hit squads and dirty tricks units was founded during the presidency of Pieter W. Botha, who sought to salvage white-minority rule in South Africa with a combination of partial political reforms and a massive security machine designed to crush resistance at home and destabilize any country in the region that represented a potential springboard for black liberation movements.

Botha built an extensive military-security network, run by an establishment of "securocrats" who had the president's ear and came to dominate the civilian administration.

Since he became president just one year ago, de Klerk has moved to reduce the power of the securocrats and reestablish the authority of the civilian administration, but with his political constituency in turmoil as a result of his rapid reform initiatives, he has moved cautiously and avoided outright confrontation with the security establishment.

From what has been revealed by investigative reporters and the Harms Commission, a government-ordered inquiry into the existence and activities of police death squads that targeted anti-apartheid figures, the Civil Cooperation Bureau included senior army and police officers, former commandos from the Rhodesian and Portuguese colonial wars, ex-convicts, gangsters and captured ANC guerrillas "turned" during interrogation and now code-named "Askaris."

The bureau's known operations included assassinating a civil rights lawyer in Durban; switching another's heart pills to induce a heart attack instead of preventing one; sprinkling a sophisticated poison in the luggage of the secretary general of the South African Council of Churches, the Rev. Frank Chikane, that nearly killed him during a trip to the United States last year; putting out a "contract" to kill a white left-wing journalist; and, most bizarre of all, sending a baboon fetus to Archbishop Desmond Tutu, the Nobel Peace laureate, presumably to shock him.

As the security establishment gained in power during the Botha years, the Civil Cooperation Bureau operated increasingly beyond the control of the civil administration.

Although there has been no official disclosure, it is widely assumed that the bureau continued to support the Renamo rebels in Mozambique in defiance of Pretoria after Botha signed a nonaggression pact with Mozambican leader Samora Machel in 1984.

The bureau's units specialized in destabilization, and testimony before the commission has shown they often trained black gangsters and the Askaris to carry out their assassinations and dirty-tricks missions.

The training was done at a special police farm called Vlakplaas, near Pretoria. Other military camps farther north are believed to have been used for training Renamo guerrillas.

Mandela seemed to suggest at a news conference the other day that the attackers of the train may have come from one of these training bases. After saying the indiscriminate style of the attack resembled methods used by Renamo to cause widespread panic and social chaos, Mandela noted that none of the attackers had uttered a word during their rampage, possibly to avoid being identified as Mozambicans.

Black political sources also have noted that some elements of the security forces developed a technique during uprisings in the mid-1980s of using black surrogates to crush the resistance and regain control of townships that effectively had been taken over by ANC supporters and street committees.

They did this by exploiting political rivalries within the black community, turning group against group in recurring sequences of black-on-black violence.

In the best documented of these operations, bands of vigilantes launched a series of attacks on satellite settlements around Cape Town's Crossroads squatter camp that were known ANC strongholds.

In 1986, the vigilantes razed four satellite camps, driving an estimated 70,000 people from their homes and killing hundreds, while the security forces stood by.

Witnesses to many of the recent township attacks claim the same technique is now being used, with the "third force" moving in to aggravate the black power struggle between Inkatha and the ANC.