A letter tainted with a poisonous substance was sent to South Africa's top prosecutor, who is pursuing corruption cases against senior government leaders and politicians, his office said Monday.

Five staff members became ill Thursday after handling the letter addressed to their acting boss, Mokothedi Mpshe, said Tlali Tlali, a spokesman for the authority.

Mpshe did not touch the letter.

Tlali said an investigation was under way into the origins of the letter. He provided no information about its contents or where it might have been mailed from.

He said he could not identify the type of poison in the letter because he was awaiting the results of forensic tests that were conducted.

Staff had to be evacuated from the authority's Pretoria premises, and Mpshe's office was quarantined on Thursday.

Tlali said staff members who handled the letter developed rashes and headaches but were back at work Monday.

Mpshe is heading the prosecution of African National Congress President Jacob Zuma on corruption charges and the fraud and racketeering case against national police commissioner Jackie Selebi. His office also has been investigating several high-profile organized crime cases.