West Yorkshire's chief constable has "searched his soul" about what could have been done to prevent an officer being killed and another injured.
Colin Cramphorn has closely examined the robbery
But Colin Cramphorn said he had concluded the two officers had the right protective equipment and back-up to deal with the robbery in Bradford.
Pc Sharon Beshenivsky died and her colleague Pc Teresa Milburn was injured in a shooting on Friday.
Six people arrested at the weekend are still being questioned.
Five Somalian men and a woman, whose nationality is not known, were held after raids in London.
By 1100 GMT on Wednesday police will have to issue charges or release the five suspects.
Sharon Beshenivsky has only been a constable since February
A fifth man, who was arrested later, is still being questioned by officers.
Speaking to BBC News, Mr Cramphorn said: "I have of course searched my own soul.
"Could we have done more? Should we have done more?"
But he went on to say: "I honestly cannot at the moment find anything we should or could have done differently.
"These officers had the right protective equipment.
"These officers were given the right support and back-up from their communications room."
Pc Beshenivsky, 38, was shot in the chest during the robbery at the Universal Express office in Morley Street.
She was taken to Bradford Royal Infirmary where she was found to be dead.
Pc Milburn, 37, who was shot in the shoulder by the robbers, has now left hospital.