Police are continuing to question six people arrested over the killing of Pc Sharon Beshenivsky.
Ch Supt Sarah Brown signs the book of condolence
The 38-year-old was shot dead and her colleague Pc Teresa Milburn, 37, injured during an armed raid at a travel agency in Bradford on Friday.
Five Somalian men and a woman, whose nationality is not known, were arrested in London on Saturday.
On Monday night magistrates granted police an extra 36 hours to question four of the men and the woman.
By 1100 GMT on Wednesday they will have to issue charges or release the five suspects.
A fifth man, who was arrested later on Saturday, is still being questioned by officers.
On Monday, streets around the scene of the robbery remained cordoned off as forensic examinations continued.
Pc Beshenivsky had three children and two step-children
Pc Beshenivsky's boss, divisional commander Ch Supt Sarah Brown, described her death as the "worst nightmare" for a police chief.
Opening a book of condolence at Bradford's City Hall, Ms Brown said Pc Beshenivsky's death was a massive tragedy for the whole force.
"The mood at the station continues to be one of sombre determination as we all try to come to terms with what has happened.
"We have lost one of our colleagues and another has been seriously injured in the most dreadful of circumstances.
"We have taken some comfort from the messages of condolence we are receiving from throughout the world."
Pc Milburn, who was shot in the shoulder by the robbers, paid tribute to her dead colleague, after leaving hospital in a wheelchair.
She said: "Friday should have been a normal day at work, instead I lost a colleague and a dear friend.
"Sharon and I enjoyed working together immensely.
"Nothing can bring Sharon back and my wholehearted sympathy goes out to her family."
Travel agency owners in Bradford said the robbery was one of a growing number of raids on Pakistani-owned businesses.
Significant amounts of cash are handed in at travel agencies by people arranging money transfers for relatives in Pakistan.
Shahid Bhatti, owner of Bradford Travel on Lumb Lane in Bradford city centre, said: "Over the past 10 years a pattern has been established of robberies on travel agencies in Bradford.
"We have been targeted because people know about the money transfers.
"My brother was robbed outside our office in 1997 and since then we have increased our security. We have installed CCTV cameras and we report any unusual behaviour on Lumb Lane.
"This incident is a horrible tragedy, but we've been expecting something like this."