Kaiser's body was found seven years ago with a gunshot wound to the back of the head.
A court recently rejected an earlier finding that the outspoken priest had committed suicide.
Attorney General Amos Wako said on Wednesday that he had directed the commissioner of police "to expeditiously and comprehensively carry out further investigations as identified by the court".
The announcement from Kenya's top law enforcement official came ahead of tomorrow's seventh anniversary of Kaiser's death.
Kenya orders new probe into death of US priest (Ecumenical News International, 23/8/07)
LINKS (not necessarily endorsed by Church Resources)
John Kaiser website
Missionary priest murdered, Kenya inquest finds (CathNews, 2/8/07)
New investigation of missionary's 'suicide' (CathNews, 12/8/03)