The company tracked the white Spar lorry using a GPS system, police said.

When officers found it, the curtains in the cab were drawn and it was unlocked. They tried to resuscitate Stacey, but were unable to do so.

After searching the area, they found the stepfather hanged from a tree.

Detective Chief Inspector Kirk said police now had more details about the journey Walker made with Stacey, who is from the West Midlands area.

She said: "It (the lorry) will have spent a great deal of time in Norfolk and Cambridgeshire during daylight hours, particularly on the A47, so I'm hopeful we will get some good information about its movements during the day.

"We have got no evidence at this time to indicate that Stacey was sexually assaulted.

"While our investigation is still at an early stage, it is clearly something we would like to rule out as quickly as possible."

After leaving its depot in Willenhall in the West Midlands at 4am on Friday, the lorry headed straight for Great Yarmouth in Norfolk.

After making deliveries to Spar stores in the county, it was returning to the West Midlands, when it stopped just outside Peterborough at about 3pm on Friday. It arrived in the lay-by in nearby Warmington shortly after 3.35pm.

It is understood that his lorry was used to carry fresh goods.

But when the lorry failed to return, the HGV company contacted police.

Police said they believe the lorry was parked some time late on Friday.

Det Ch Insp Kirk added: "We believe it's all taken place in the vicinity of that lay-by, and that's our main crime scene.

"The man is part of the family set up and they have a close relationship as stepfather and child. The mother was comfortable with the child being with the male, and there were no issues or concerns raised within the family.

"Our investigation is to try and find out what led to this tragic event taking place."

The lorry has now been removed from the Jet filling station on the A605, which is close to Warmington, on the border of Northamptonshire and Cambridgeshire.

Witnesses said the lorry had been parked in a road adjacent to the filling station.