Stacey Lawrence travelled with Darren Walker in his lorry
A girl found strangled in her mother's boyfriend's lorry may have been sexually assaulted, police have said.
Stacey Lawrence, nine, was found in the cab of the lorry on the A605 near Warmington, in Northamptonshire, on Saturday afternoon.
Darren Walker, 40, from West Bromwich, was found dead in woods nearby.
Det Ch Insp Tricia Kirk of Northants Police said: "We cannot rule out now that there was some sexual touching involved in this incident."
She added: "We would like nothing more than to be able to give the family a clear picture into what has gone on.
"The tests we are doing we are hoping may give us a clearer indication. But we may never know exactly what happened in that lorry."
Det Ch Insp Kirk said Mr Walker had a history of a domestic incidents with his wife of three years, who was in the process of divorcing him.
He was cautioned for an assault on his wife in 2006.
Northamptonshire Police, working with detectives in the West Midlands, are working to track down Mr Walker's previous partners to uncover any other incidents, and the family computer has also been seized.
It is believed Mr Walker murdered Stacey, also of West Bromwich, then hanged himself from a tree.
Det Ch Insp Tricia Kirk said 'sexual touching' could not be ruled out
Det Ch Insp Kirk added: "She was strangled with a ligature of black fabric, which is the same fabric as Walker used to hang himself.
"It's some sort of strapping, we're trying to identify what it's actually used for."
Mr Walker worked as a lorry driver for wholesale distribution firm AF Blakemore and Son, which delivers goods to Spar shops across the Midlands from a depot in Willenhall, West Midlands.
Stacey had accompanied him on a trip to make deliveries in East Anglia.
Police said the vehicle left the depot at 0400 BST on Friday and headed for Great Yarmouth in Norfolk.
'She trusted him'
After making deliveries to Spar stores in the county, it was returning to the West Midlands when it stopped just outside Peterborough at about 1500 BST.
It arrived in the lay-by in nearby Warmington shortly after 1535 BST.
Det Ch Insp Kirk said the pair were spotted leaving the lorry to use a toilet at the services.
Stacey's mother Roxanne last heard from Mr Walker some time between 1930 and 2000 BST that night.
She said: "She trusted him and did not have any reason to think that he would ever cause any harm to her daughter.
"When he called her, he said he was about to go to bed and was expecting to leave in the early hours of Saturday to get back to the depot.
"She heard Stacey talking in the background because she was watching a programme that she particularly liked.
It was a model family, very well balanced, very popular, the parents were very supportive and we can't understand what's gone on here, really
Keith Shilton, Stacey's head teacher
"Her mother thought that was My Family.
"She doesn't describe there being any indication that there was anything wrong, it was just a normal conversation."
In a newspaper interview, Ms Lawrence said she had been planning to marry Mr Walker, who she believed was a dedicated family man.
Keith Shilton, head teacher of Eaton Valley Primary in West Bromwich, described Stacey as one of its star pupils.
"We are devastated. None of us can understand what's gone wrong," he said.
"It was a model family, very well balanced, very popular, the parents were very supportive and we can't understand what's gone on here, really."
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