Two private detectives claim they know who killed Stellenbosch student Inge Lotz and why, and have laid charges of intimidation and obstruction of justice against two police officers who they claim knew the details and did nothing about it.

Daryl Els and Christian Botha, private detectives appointed by the family of Lotz's boyfriend, Fred van der Vyver, said a witness to the murder had made a second statement to them recently in the presence of his lawyer.

The alleged witness is currently in jail on drug and murder charges.

On Tuesday Botha said that he and Els had laid charges at the Stellenbosch police station on Monday.

He said they had known about the witness and what he had said before Van der Vyver's murder trial had begun and that policemen who also knew about it at the time had done nothing to investigate further.

Lotz was brutally murdered in her Stellenbosch flat on March 16, 2005.

Last month, Van der Vyver was found not guilty of her murder in the Cape High Court by Judge Deon van Zyl and two assessors, who slammed the police investigation into the case.

Last week, private detective Niel van Heerden, of the firm George Fivaz and Associates, also laid charges of obstruction of justice and perjury against top investigators involved in the case.

Els and Botha, as well as Van Heerden, have accused members of the police team of lying under oath and of withholding information pertinent to the case from the court.

Els told the Cape Argus on Tuesday that according to the witness three killers had entered Lotz's apartment on the day of her death. One person was an acquaintance of hers.

"According to the witness … there were six people in total, but three entered the flat," he said.

"We have reason to believe their motive was to get money for drugs. All six are known to us and most of them are in jail at the moment waiting to appear on similar charges of murder, robbery and drug dealing."

Els and Botha also claim that the witness had been visited in jail by two police officers shortly after Van der Vyver was charged and told to "change his story" or face the consequences.

That was after he had already made a statement to them in 2005 saying he was involved in the murder.

Els said the reason why nothing was found to have been stolen from Lotz's flat at the time of the murder was that something had gone wrong and that the killers had fled before they could take anything.