The police officer who investigated the mercy killing of Sarah Lawson was a neighbour who had watched her grow up.

Detective Inspector Paul Williams lived in the same street where 22-year-old Sarah, who was mentally unstable, was killed by her father Jim.

On the morning she died, after Lawson gave her a cocktail of drugs and suffocated her with a pillow, Mr Williams was phoned at his home in Georgia Avenue, Worthing.

He said: "It was very early in the morning. I got up, got dressed and walked to the house.

"Everyone was there; Jim, Sarah's mother Karen and brother Jamie.

"Although I do not know the family well we have a lot of friends in common.

"I used to see Sarah and her brother walking to school together and saw her grow up over the years."

Father-of-four Mr Williams has lived in the street for 17 years.

He attended Broadwater CE Primary School, where Sarah was later a pupil.

He believes a judge would be wrong to send her father to prison.

Mr Williams, who has been with Sussex Police for more than 20 years and has a psychology degree, said: "Someone is sent to prison as punishment.

"The family has already been punished enough and Jim Lawson has never been a threat to society. He is a hard-working, decent man.

"The only other reason to imprison him would be to send a message to society and deter others but Jim's situation is so rare surely that does not apply."

On Monday Lawson, 52, pleaded guilty at Maidstone Crown Court to manslaughter on the grounds of diminished responsibility. He will be sentenced a week on Friday.

The court heard how Sarah, who had tried to kill herself several times that week, had spoken with her father about the pain she and her family were suffering as a result of her illness.

Lawson said he had spoken to his daughter about killing her and they both decided it was "the best thing to do".

Mr Williams said: "The case speaks for itself. There is a limited number of words to describe it. Poignant, awful, tragic - they all fail.

"Any difficulties I had with dealing with this because I knew the family from a distance pale into insignificance in comparison with what they have been through."