A judge today rejected an application to free a white former police captain allegedly involved in a secret terror network that harassed and assassinated anti-apartheid activists. The suspect, Ferdinand Barnard, has been held without charge since Oct. 31 as part an investigation into the assassinations last year of two white activists: Johannesburg university lecturer David Webster and Namibian lawyer Anton Lubowski. Pretoria Supreme Court Justice H.J. Priess rejected a bid by Barnard's father, a retired police colonel, to have the former narcotics detective released. In Windhoek, Namibia, another Supreme Court judge today set April 18 as the start of a murder trial for Donald Acheson, a suspected accomplice of Barnard who is accused of involvement in Lubowksi's killing. Lubowski, shot and killed outside his Windhoek home on Sept. 12, was the only white to hold a leadership position in Namibia's independence movement, the South-West African People's Organization. Webster, a well-known human rights activist, was shot and killed outside his Johannesburg home on May 1. Police Brig. Floris Mostert, in testimony opposing Barnard's release, said the investigation into the assassinations had uncovered the existence of ``a secret organization ... with members from all levels of society, which strives to terrorize left-wing radicals with the aid of violence and intimidation.'' Dozens of anti-apartheid activists have been the victims of unsolved murders in recent years. Many others have had their cars vandalized, and there have been bombings or arson attacks on offices of militant trade unions, civil rights groups, and opposition newspapers. Mostert said Barnard had been in contact with Acheson prior to Lubowski's murder. The brigadier also said: ``I must respectfully stress that the nature of my investigation is extremely sensitive and that the secrecy of the information which I possess is of the utmost importance.'' Police say the Webster-Lubowski case is not directly related to investigations of claims that death squads within the ranks of the national police force killed government opponents on the orders of high-ranking officers. One black ex-policeman, Butana Nofemela, has pleaded guilty to committing a murder as part of such a death squad, and a white former captain, Dirk Coetzee, has fled abroad after telling a newspaper he commanded a death squad. Thus far, legal steps have been taken only against Nofemela and Coetzee, not against other policemen implicated by them. This has resulted in widespread demands for an independent judicial inquiry and accusations that President F.W. de Klerk's government is engaged in a cover-up. Some of the white officers implicated by Coetzee appeared today at an inquest into the deaths of four young black activists in 1986. The policemen have denied allegations that the youths were murdered, saying they were shot in self-defense by a special counter-insurgency unit.