Detectives hunting the killers of Pc Sharon Beshenivsky have named the three prime suspects they want to question.
The 38-year-old officer was shot dead during an armed raid by a gang at a travel agency in Bradford a week ago.
The suspects are Mustaf Jama, 25, from the north Kensington area of London, and his brother Yusuf, 19, from the same area. Both are of Somali origin.
They also want to trace Muzzaker Shah, 24, who was born in Burnley but has lived in London since 2001.
'Still in UK'
Det Supt Andy Brennan, who is leading the hunt for the killers, said: "I believe the men who committed this crime are very dangerous." They were already "known to police", he added.
He told reporters at a briefing in Bradford there was no indication any of the three suspects had left the country. "It is possible they may be in the London area," he said.
"I am appealing for any information which can help us to trace these men as a matter of urgency."
Mr Brennan also gave more information about the movements of the getaway car believed to have been used by the killers.
He said: "Immediately after the events at the travel agents three men were seen running to a silver vehicle which was parked in Howard Street.
"We believe this vehicle is the silver Toyota Rav 4, registration WP05 YTT, details of which were provided to the media previously.
"We have trawled through many hours of CCTV and have a number of sightings of the vehicle on 18 November [the day of the shooting].
"This includes a sighting of the vehicle travelling along Morley Street at 3.02pm, moments before the events at the travel agents."
He said the vehicle was seen 40 minutes later travelling along Manchester Road, out of Bradford city centre, in the general direction of the motorway network.
Meanwhile, the people of Bradford have stood united to observe a two minute silence during a civic memorial service for Pc Beshenivsky.
The Bishop of Bradford, the Rt Rev David James, and Force Chaplain Sam Sagar led the public service in the memorial gardens at Centenary Square.
"In a place near here at this time last week there was an act of unimaginable evil that brought great darkness to this city and to our nation," said Mr Sagar.
"We pray today for Sharon's family, we ask that in their grief and in their sorrow they would feel comfort in feeling your presence with them."