Tracy Mertens, 31, from Rochdale, Greater Manchester, was found barely alive by a local resident on the steps of a church in Eaton, Cheshire, on Friday afternoon.
She was taken to North Staffordshire Hospital but died early on Christmas Eve from extensive burns, possibly as severe as 100 per cent.
A post-mortem examination confirmed that the burns were the sole cause of death, discounting earlier reports that she had also suffered broken limbs.
Detectives from Cheshire, West Midlands and Greater Manchester were liaising in the hunt for the killers but said yesterday afternoon that they still had not established a motive for the murder.
Ms Mertens was abducted from a terraced house in Birmingham on Friday morning and driven north to rural Cheshire where she was murdered later that day.
They were still searching for two black men, believed to be aged about 30, who were seen driving her away from the house in what was described as a "tatty'' yellow Ford Escort.
A police spokesman said that many of the Midlands' roads were extremely busy with pre-Christmas traffic on Friday, including the M6 which is monitored by numerous security cameras.
Police have appealed for anyone who might have seen the car or its occupants to come forward with information.
"There are still no indications as to what the motive could have been and we still want to hear from anyone who might have seen this car between 11am and 4.20pm on Friday between Birmingham and Eaton,'' a police spokesman said yesterday.
Ms Mertens, whose children were being looked after by relatives yesterday, lived in Rochdale but spent a lot of time in Birmingham where her partner had several relatives.
Police and forensic experts were at the Birmingham house yesterday while Cheshire detectives liaised with colleagues in Manchester.
The villager who found Ms Mertens said that she was barely conscious when she found her slumped next to the church in Eaton.
"She was in a terrible state. Someone else from the village stayed with her while I rang for the police and ambulance,'' she said.
Five men were arrested in Glasgow yesterday and charged with the murder of Gary Talbot, an 18-year-old student who died in the city in the early hours of Christmas morning.