Police may never know exactly why Derrick Bird launched his massacre, they have admitted.
Detective Chief Superintendent Iain Goulding, of Cumbria Police, said he and his officers were "absolutely determined" to get to the bottom of why the tragedy happened.
But he conceded: "However, it may not be possible to establish all the answers because we cannot speak to Derrick Bird."
The detective said it appeared the taxi driver selected some of his victims deliberately and others randomly.
He said: "A key part of the 'why' in this investigation is trying to establish whether those tragically killed were chosen because of a motive, because of a grudge or were simply random killings. Our initial assessment is we have a combination of both. But I'm not speculating further at this time."
Mr Goulding said Bird had never been to prison and police had found no record of any mental health problems, contact with mental health agencies, or medication.
But he refused to comment on the killer's motives for the shootings and ignored questions from journalists about whether he left a suicide note.
He said: "We are aware there is much speculation about the motive behind this crime. We hear rumours of finance and domestic. We are aware of those things and they are all proper lines of investigation for us, but we will not be speculating."