The central allegation has been that this Third Force has deliberately orchestrated the political violence in the black townships - which has claimed more than 7,000 lives in the past two years - using as its instruments both serving security force members and Zulus of the Inkatha Freedom Party.
The tactical objective of this 'low-intensity war', it has been said, has been to destabilise the African National Congress, by far the biggest political grouping in South Africa and as such the greatest obstacle to a strategy aimed at the perpetuation of white political control after apartheid.
What strengthens the case is that the finger of suspicion once again points to General Christoffel van der Westhuizen, head of South African Defence Force military intelligence (known formally as CSI, Chief of Staff Intelligence).
It strengthens the case because to understand what is happening today in the townships it is necessary to know the history of those who pose a secret threat to democracy in South Africa. In that history, General van der Westhuizen occupies an interesting place.
Between 1978 and 1989 P W Botha ran the country through his generals. Real power resided not in parliament but in the State Security Council (SSC), a body dominated by military intelligence and the security police. All significant decisions of state were subordinated to the counter-insurgency imperative, which meant stopping the ANC and its allies from taking power.
Inside South Africa, the state hit upon the brilliant idea of adapting the surrogate method - using blacks to fight blacks on the white man's behalf - against the United Democratic Front, the ANC by another name at a time when the ANC was illegal.
Hit squads, manned by white military personnel, offered an alternative 'total strategy' method. The existence of the CCB, the Civilian Co-Operation Bureau, was revealed in 1989, a newspaper exposure that led Mr de Klerk to appoint the Harms Judicial Commission of Inquiry to investigate allegations of state murder. In early 1990, Mr de Klerk promised the commission would 'cut to the bone'.
In 1985 the officer commanding the SADF's Eastern Province Command was Brigadier van der Westhuizen. On 8 May this year New Nation, a Johannesburg newspaper, published a copy of a top-secret 'signal message' sent to the SSC on 7 June 1985 from the Eastern Province Joint Management Centre. The message, whose authenticity has been confirmed, contains details of a telephone conversation between Brigadier van der Westhuizen and a General Van Rensburg, a senior member of the SSC secretariat. Three names are mentioned: Matthew Goniwe, Mbulelo Goniwe and Fort Calata. The document says: 'It is proposed that the above-mentioned persons are permanently removed from society, as a matter of urgency.'
On 27 June 1985, Matthew Goniwe (a powerful UDF leader), Fort Calata (another prominent UDF leader) and two political associates, Sparrow Mkonto and Sicelo Mhlauli, were forced out of a car in which they were driving then assassinated. Their four bodies were found some days later, charred, stabbed and mutilated.
Today Christoffel van der Westhuizen is a general. In November last year he was appointed Chief of Staff Intelligence of the SADF. Before that he had been the SADF commanding officer in the Johannesburg area. It is possible, it was reliably learnt yesterday, that General van der Westhuizen will be asked to testify at an inquest into the assassination in Johannesburg on 1 May 1989 of ANC activist David Webster, widely thought to have been the victim of a CCB hit squad.
President de Klerk has reacted with great shows of indignation to accusations by Nelson Mandela in recent weeks that he personally has been spearheading the alleged security force campaign to orchestrate violence.
His indignation is not entirely forced. The evidence of today's revelations reinforces the conclusion, reached by previous Independent investigations, that he is not in charge of these sinister SADF elements. It is inconceivable, after all, that he would have sanctioned the plan to kill Mr Coetzee.
Behind the scenes, and completely unacknowledged by Mr de Klerk, a battle has been going on between his allies in the National Intelligence Service on the one hand and leading figures in military intelligence and the security police on the other. Mr de Klerk also has the support of 'conventional' military men in the army, navy and air force.
The most plausible theory put forward as to what is happening is that the most senior officers in the security forces, while sharing Mr de Klerk's key objective of holding on to white power after apartheid, doubt that he alone, employing conventional political methods, can do it. He needs the help of the dirty tricks brigade, the killers and the manipulators, to defeat the ANC and win the day.