The identities of two prime suspects in the shooting of Pc Sharon Beshenivsky have been circulated to police forces nationwide, the BBC has learned.
Pc Sharon Beshenivsky joined the Bradford South team in May
Detectives confirmed they were now actively seeking three men after significant developments in the case.
Pc Beshenivsky was shot dead and her colleague, Pc Teresa Milburn, injured during a robbery at a travel agents.
Police also issued details of a silver Toyota hire car thought to have been used in Friday's robbery in Bradford.
Five Somali men, and a woman, were arrested in London over the weekend and brought to police stations in West Yorkshire but have all now been released.
BBC correspondent Mark Simpson said it is understood police know the names of two of the men regarded as prime suspects in the case.
Their names, which have not been made public, are being circulated to police forces and to airports.
Detectives investigating the shooting said they had identified a car, believed to have been used in the robbery, as a Rav 4 model, registration number WP05 YTT.
Det Supt Andy Brennan, of West Yorkshire Police, said: "The vehicle was hired at Heathrow Airport on 25 October.
"We believe the vehicle travelled to West Yorkshire from London in the early hours of Friday and returned to London later that evening."
Police now have possession of the Toyota and want to hear from anyone who may have seen it particularly last Friday, the day of the robbery.
Police say the Rav 4 was hired at Heathrow airport on 25 October
Pc Beshenivsky, 38, died after being shot in the chest on her youngest daughter's fourth birthday. She had been a serving police officer for nine months.
Her colleague Pc Milburn, 37, who joined the West Yorkshire force in April 2004, was shot in the shoulder during the raid on the travel agents on Morley Street in Bradford.
Metropolitan Police Commissioner Sir Ian Blair said Scotland Yard detectives were helping West Yorkshire Police.
He said significant developments were expected in the next 24 hours and the search for suspects was being extended wider than the Somali community.
At Prime Minister's Questions in the Commons, Tony Blair offered condolences to the family of Pc Beshenivsky who, he said, had been "so tragically murdered on duty".
A civic memorial service for Pc Beshenivsky will be held at Centenary Square in Bradford on Friday afternoon.
The service will be led by the Bishop of Bradford, the Rt Rev David James, and include a two-minute silence.
Meanwhile in Bradford, a large police operation was still continuing at the scene of the shooting with officers carrying out house-to-house inquiries in the city.
Outside the travel agency, forensic scientists have been carrying out a fingertip search behind a wooden barricade.