A fifth person was arrested on Thursday for allegedly plotting to kill a police investigator associated with the Radovan Krejcir investigation, said Gauteng police.

"The suspect is a 34-year-old woman, who is a sergeant within the crime intelligence environment," said Lt-Gen Solomon Makgale.

He said the woman allegedly provided software to track investigators.

Nine cellphones were confiscated from her.

Earlier, three men and a woman were arrested for allegedly plotting to kill forensic investigator Paul O'Sullivan and crime intelligence officer Colonel Nkosana "Killer" Ximba.

One of the men was also linked to the kidnapping and attempted murder case for which Krejcir is being investigated.

The kidnapping case allegedly involves Czech fugitive Krejcir, Sandton businessman Desai Luphondo, and two members of the Hawks - Warrant Officers Samuel Modise Maropeng and George Jeff Nthoroane.

They were arrested in November.

In June, they allegedly kidnapped and assaulted a man whose brother, known only as "Doctor", disappeared with a 25kg shipment of tik he had been recruited to help smuggle through OR Tambo International Airport.

They allegedly kept the man at Krejcir's Money Point business in Bedfordview, Johannesburg, for four days, and assaulted him.