The police watchdog is investigating a 999 call made on the day a family of four were murdered in their own home.
University lecturer Jifeng Ding, his wife Helen, and their two daughters were found stabbed to death in their home in Wootton, Northampton, on Sunday, May 1.
It is believed they were killed on Friday, April 29.
Detectives are hunting businessman Anxiang Du, 52, who has not been seen since the day of the murders.
It is thought he may have travelled to London in a Vauxhall Corsa that went missing from the Ding family home and was later found in a side street in St John's Wood in the capital.
Northamptonshire Police yesterday confirmed a 999 call was made in connection with the case at 3.31pm on Friday, April 29.
A spokeswoman said they had referred the matter to the Independent Police Complaints Commission (IPCC) but could not comment any further.
The IPCC confirmed it is investigating "one aspect" of Northamptonshire Police's involvement in the case.
A spokesman said: "The IPCC can confirm we are independently investigating one aspect of Northamptonshire Police involvement in this case.
"With police efforts properly focused on the murder investigation and tracing a suspect in the case, we are unable to give further information at this stage."
Earlier this week police said Crimestoppers has put up a reward of up to £10,000 for information leading to the arrest and conviction of the person responsible for the murders.
The case is also due to be featured on BBC Crimewatch on Tuesday.
Police yesterday also released new CCTV images of Mr Du, all from Friday April 29.
The pictures show the 52-year-old at Coventry railway station at 10am as he was on his way to Birmingham.
He is then seen at Birmingham New Street station at 10.50am and the Pavilions Centre, Birmingham, at about 11.20am.
The final two images are of Mr Du at Northampton bus station at around 12.50pm and show him without his baseball cap.
Det Supt Glyn Timmins repeated his appeal to anyone who saw Mr Du travelling to Wootton in Northampton on April 29 to come forward, adding: "Any information, however small, could be of great help as we build the picture of what happened that afternoon."