A businessman suspected of murdering a family of four travelled to the victims' home just a day after losing a bitter court battle over their shared business interests, it emerged today.
Northamptonshire Police said Anxiang Du, who has still not been traced, was now known to have caught a train from Birmingham to Northampton last Friday, when the family are thought to have been stabbed to death.
Mr Du, 52, was named as a suspect yesterday by detectives investigating the murders of university lecturer Jifeng Ding, his wife Helen and their two daughters.
Releasing CCTV images showing Mr Du at both Birmingham New Street and Northampton railway stations, Detective Superintendent Glyn Timmins confirmed that the businessman felt he was owed tens of thousands of pounds by Mrs Ding.
Addressing a press conference, Mr Timmins said officers had also established that Mr Du caught a bus from Northampton to Wootton, where the Ding family lived.
Explaining the links between the suspect and the victims, Mr Timmins said the business relationship between Mr Du and Mrs Ding was being explored as the "primary motive" for the killings.
The detective added: "Clearly there was a dispute over a considerable amount of money related to a business that ended up in the civil courts.
"I think there was a suit and a counter-suit and at some stage I believe that the judgment went against Mr Du, or at least he hadn't received the money that felt he was due.
"That is the motivation that we are exploring."
Mr Timmins, who confirmed that the final court activity was thought to have taken place last Thursday, said Mr Du, who was carrying a yellow Adidas rucksack, was seen at New Street station at 11.22am on the day of the royal wedding.
Further footage showed that he was in Northampton by 12.35pm and the family, whose bodies were found in separate rooms, are believed to have been killed later that afternoon.
After Mr Du caught a bus to Wootton, his movements are unknown and police are examining whether the Corsa was driven away from the Northampton area.
Mr Du, who lived in Coventry and worked in Birmingham, was reported missing by his family on Friday after they discovered a suicide note at the shop where he worked.
Dr Ding, who worked at Manchester Metropolitan University, his wife, a part-time teacher, and their daughters all died from stab wounds.
They were found dead at their detached home in Pioneer Close, Wootton, on Sunday and police later established that a Vauxhall Corsa hired by one of the victims was missing from the address.
The car has not been traced and officers admit that they are puzzled that it has not been seen since or triggered cameras on the road network which record number plates.
Chinese-born Mr Du is described as being of slim build and routinely wears a baseball cap.
Members of the public are being advised not approach Mr Du and anyone who spots him or the silver five-door Corsa, which has the registration BG60 PMO, is urged to call 999.
The scene of the murders remains cordoned off and police, who have yet to find the murder weapon, were still searching nearby drains.
In excess of 60 officers are now working on the investigation, although that number is likely to increase as the inquiry widens, Mr Timmins said.
The detective added: "We do not believe that these murders were gang-related and are working on the assumption that the motive relates to business associations and a financial dispute.
"We believe that Mr Du could be anywhere in the country, and at this stage there is nothing to indicate that he may have left the country or indeed that he even has his passport with him."