Det Ch Insp Lucy Edgeworth, of the major crime investigation team, said “all the evidence suggests” the murder was a “pre-planned, cold-blooded execution”.
Saying the motive remained unclear, she added: “We know there was a dispute about rent. We know that they held different political views.
“But none of those things make any sense to why somebody would carry out such a horrific crime.”
The expert explained: “The retrieval of the clothing from the pond, again, all forensically linked Zach to that crime. So it was a really interesting and complex forensic picture.”
Ms Edgeworth added: “Jamie’s family and friends have been put through an ordeal that is impossible to adequately describe.
“They have experienced the most appalling tragedy and lost a son and a brother who clearly meant the world to all of them.
“They have shown remarkable levels of courage during our investigation and trial, watching footage that no family should ever have to sit through.
“Today’s conviction sadly does not undo any of the pain they continue to feel, but hopefully it helps them going forward to know what happened to Jamie.”