Rapport newspaper cited the letter on Sunday in a report on apartheid-era atrocities allegedly committed by African National Congress leaders.
It said the letter contained an instruction from Ackermann to police that more evidence should be gathered on the matter, Rapport said.
"However, in the past week police refused to say whether the instruction was obeyed, and what progress has been made," Rapport reported.
It added that police had done nothing to follow up cases involving 37 ANC leaders, and that dockets compiled on this had vanished.
