Six people arrested in London in connection with the murder of a female police officer in Bradford, West Yorkshire were taken to the northern county today for questioning.
Sharon Beshenivsky, 38, a mother of three children and two step-children, was shot dead on Friday after she responded to an alarm at a travel agency in the centre of Bradford.
Her colleague Teresa Milburn, 37, was shot in the shoulder and is still in hospital under armed guard.
Police in London arrested five men and one woman in connection with the case yesterday.
The murder has dominated the front pages of British newspapers and sparked debate over whether police officers should be armed.
British police do not normally carry firearms, although small specialist units do.
"Even if routine arming is not correct, we do believe that the number of authorised officers remains too low," said Jan Berry, head of the Police Federation, the body that represents all British police officers.
The government rejected a call for all officers to be armed, and Chief Constable Colin Cramphorn, head of the force for which Beshenivsky worked, said arming police would raise as many problems as it intended to solve.
But Britain's former top police commander said the case had forced him to rethink his views on capital punishment.
"All my life I've been against the death penalty. But after the cold-blooded murder of policewoman Sharon Beshenivsky, I've changed my mind," John Stevens wrote in Sunday's News of the Worldnewspaper.
"I genuinely never thought I'd say this, but I am now convinced that the monster who executed this young woman in cold blood should, in turn, be killed as punishment for his crime," said Stevens, who retired in January.
Eighty-nine police officers have been killed on duty in Britain in the last 30 years.
The last woman police officer to die on duty was 29-year-old Alison Armitage, who was run over by a stolen car during an undercover operation near Oldham in 2001.
The 1984 death of 25-year-old policewoman Yvonne Fletcher after shots were fired from the Libyan embassy in London led to Britain severing diplomatic relations with Libya for 15 years.