One of the country's most wanted men is believed to have lured two unarmed female police constables to their deaths.
Greater Manchester Police (GMP) Chief Constable Sir Peter Fahy said Dale Cregan is thought to have made a bogus burglary report then attacked the officers, opening fire with a gun and a grenade.
Sir Peter named the "brave" officers as Fiona Bone, 32, and Nicola Hughes, 23.
He said: "It would appear Cregan has deliberately done this in an act of cold-blooded murder." He added that Cregan's motive for the attack was "impossible to fathom".
After the attack, the fugitive, already wanted in connection with separate gun and grenade attacks that killed a father and son, gave himself up at Hyde police station in Greater Manchester.
Sir Peter paid tribute to Miss Hughes, describing her as a "chatterbox" and a "great bobby" who was "always smiling". He said Miss Bone was a "calm, gentle woman", an "excellent bobby" and had been in the middle of planning her wedding.
Speaking at a press conference at GMP headquarters, Sir Peter said the force was "absolutely devastated" by the shootings, adding: "We are all shocked by what happened." He said there was nothing in the burglary report to suggest that armed police would be required.
Sir Peter said he believed Cregan had been "protected by a criminal conspiracy to harbour him", adding that the force was "fully determined" to investigate that conspiracy and bring those involved "to book".
He added: " This is one of the darkest days in the history of the Greater Manchester Police, if not for the police service overall, because we have lost two deeply-loved and valued colleagues, because they are part of our team, policing is very much a family."
Prime Minister David Cameron said that the killings were "a shocking reminder of the debt we owe to those who put themselves in danger to keep us safe and secure" while Deputy Prime Minister Nick Clegg said the officers were "victims of an appalling crime".