Although the opening victim shown is intended to appear nude, a pair of white panties can be seen on the edge of frame as she steps into the shower; also, she is obviously wearing a grey swimsuit or bodysuit when in long shot.
When McGarrett was going after the killer on the crane, both were obviously stuntmen. The killer had much longer and bushier hair as did McGarrett's double. McGarrett's double also appeared to have a mustache.
McGarrett waits alone for the killer to enter Ms Klane's bedroom, and his quarry escapes as a result. This curious lack of manpower serves no purpose other than setting up the lengthy chase and aerial stunt sequence which ends the episode (directed by a former stuntman).