A teenage girl whose body was found in a field was said last night to have been lured into a meeting by a man posing on the internet as a 16-year-old boy.

The body of Ashleigh Hall, 17, was located on Monday after police were led to the field in Co Durham by a 32-year-old man who had been arrested for traffic offences on the A177 near Sedgefield. The man was reported last night to be a convicted sex offender who met Ashleigh through the social networking website Facebook. He is said to have claimed she suffocated after he tied up, gagged and threw the girl into the boot of his car.

Ashleigh had left her home in Darlington on Sunday, telling her mother she was staying overnight with a friend.

Download the new Independent Premium app Sharing the full story, not just the headlines

There was no obvious sign of sexual assault and the cause of death is yet to be determined but Detective Chief Inspector Paul Harker confirmed officers had launched a murder inquiry. He added: "This is a very tragic case, it came about from a strange set of circumstances. A man was arrested for minor traffic offences who then told the police about the scene.

"This is a very, very unusual event. My message in terms of meeting people from the internet is please do not do it unless you are absolutely certain it is safe."

Ashleigh left Hurworth School in Darlington last year after finishing her GCSEs. Eamonn Farrar, the school's chief executive, said he was "absolutely stunned" at the death of "our Ashleigh". He added: "Ashleigh was the nicest kid in the universe, you'd never meet a nicer kid. She'd always have a smile for you walking down the corridor, she was very polite and no trouble at all."

A spokesman for the Child Exploitation and Online Protection Centre advised parents to monitor how their children use the internet. An NSPCC spokesman said: "It is not a good idea to arrange to meet people that you have chatted to online, as you can never be sure they are who they say they are."

The girl's computer will be analysed by cyber experts to find out more about her online contacts.