A man who murdered his wife and three children in a so-called honour killing feels "totally justified in what he did", police involved in the case say.
Arshad was jailed for life for killing his wife and three children
Rahan Arshad, 36, beat his family to death with a bat at their home in Turves Drive, Cheadle Hulme, Greater Manchester, in July 2006.
He then fled, leaving their bodies to lie undiscovered for almost a month.
Det Supt Martin Bottomley said Arshad felt his wife brought the killing on herself by having an affair.
The full story behind the murders is the focus of a BBC documentary, Honour Kills, to be shown on BBC Three on Tuesday.
Uzma and the children's bodies were not found for a month
It shows the work of detectives as they traced Arshad for the murder of his family.
The taxi driver had told friends and neighbours the family was going on holiday to Dubai, but it emerged during his trial that he had actually booked himself on a flight to Thailand.
The documentary crew followed detectives through every stage of their investigation, from the discovery of the bodies to Arshad's arrest and conviction.
The crew also filmed the moment Arshad confessed to the murders.
He was jailed for life in March after being found guilty of murdering his wife Uzma Rahan, 32, and children Adam, 11, Abbas, eight, and Henna, six.
The judge recommended that he stay in prison for the rest of his life.
Det Supt Martin Bottomley, senior investigating officer for the murder, said: "This documentary crew were with us for the whole of this murder investigation and the final documentary shows the hard work my team did.
"This is one of the most brutal and devastating murders I have ever dealt with.
"All of the officers who worked on this, including myself, have been affected by the depravity of this crime.
"Many of us have children, which makes the brutality he used to kill his own children even harder to comprehend.
"What is even more horrifying is that we now know Arshad feels totally justified in what he did and believed that Uzma brought this on herself and her family through the shame of her affair."
Honour Kills will be broadcast on BBC Three at 2100 BST on Tuesday.