Two unarmed female police officers have been killed in a gun and grenade attack in Greater Manchester, which led to the arrest of a wanted man.
PC Nicola Hughes, 23, and PC Fiona Bone, 32, were called to Abbey Gardens in Mottram to investigate what turned out to be a false report of a burglary.
Police said it appeared they had been deliberately "drawn" to the scene.
Dale Cregan, 29, has been arrested in connection with the officers' deaths and two previous murders.
He was detained when he walked into Hyde police station a short time after the incident.
Greater Manchester Police Chief Constable Sir Peter Fahy said someone had made a call alleging a burglary had been committed and when the officers arrived they were attacked with a gun and a grenade.
He said: "We believe he has made the call, or somebody else has made the call, and it would appear for the purposes of drawing these two officers to that particular scene, and then he has come out and shot and killed them both.
"Certainly it would appear to be that he has deliberately done this in an act of absolutely cold-blooded murder."
A witness at the scene in Tameside reported hearing 13 gunshots and an explosion shortly before 11:00 BST.
Sir Peter said police believed Mr Cregan had spent the night at the house in Abbey Gardens before the call about the burglary was made.
"This address was not known to us, was not in our intelligence systems [and] had not featured in this particular inquiry," he said.
"As would be routine, two unarmed officers were sent to the scene. When they arrived, it appears Cregan emerged into the road and killed these two officers.
"A firearm was used and a grenade was also used. We are unsure of the cause of deaths until post-mortems are carried out.
"Cregan then went to a local police station and handed himself in."
Sir Peter said the officers exemplified the very best of British policing.
He said Miss Bone was a "calm, gentle woman", an "excellent bobby" and had been in the middle of planning her wedding.
'Shocked and distressed'
He paid tribute to Miss Hughes, describing her as a "chatterbox" and a "great bobby" who was "always smiling".
"We are absolutely devastated by this loss and our thoughts and condolences go out to the families of these two officers, their friends and in particularly their colleagues who work with them day in, day out who are shocked and distressed at the events of this morning," he said.
Police said one of the officers died at the scene and the second was critically injured and died afterwards.
A statement from Tameside Hospital said its emergency team "strived to save the life of a female officer who was brought to the A&E unit with critical injuries" but that she sadly died.
The scene in Mottram has been cordoned off and there is a heavy police presence in the area, including a bomb disposal team.
A police helicopter is also on patrol overhead.
The witness who described the shots, a window cleaner who worked in the area, said the property the officers were called to had been unoccupied for some months.
'Long, dark shadow'
The BBC's North of England reporter Danny Savage said he had spoken to an eyewitness who said they saw a police car outside the house shortly after 10:15 BST.
"After that car had drawn up it seems the two unarmed police officers inside the car went into the property which had stood empty for some months," he said.
"Whilst they were inside around a dozen shots were fired and there was an explosion.
"Those police officers didn't come out of the house again, the police car was left outside with its lights flashing but empty. Emergency services and colleagues were soon here offering assistance."
Home Secretary Theresa May said: "This is a deeply shocking incident and a terrible reminder of the risks that police officers face every day to keep our communities safe."
Shadow home secretary Yvette Cooper said it was "deeply distressing news [and] a painful demonstration of how police officers put themselves in harm's way every day to protect the public".
Assistant Chief Constable Garry Shewan said it was "a tragic day" for Greater Manchester Police.
"A long and dark shadow has been cast across Greater Manchester and my thoughts are with the officers' families, friends and colleagues," he said.
Police had been offering a £50,000 reward for information leading to Mr Cregan's arrest as he was wanted over attacks that left a father and son dead.
Sir Peter said looking for him had been a "top priority" for GMP.
He said: "We have some of our best people on this operation.
"We believe that he has been protected by a criminal conspiracy to harbour him and we are absolutely determined to fully investigate that conspiracy and bring the people involved to book."
Ian Hanson, chairman of the Greater Manchester Police Federation, said: "What we've got are two young girls who went out this morning and they've got an absolute right to come home to their loved ones.
"This is cold-blooded murder, the slaughter of the innocents."
Jonathan Reynolds, Labour MP for Stalybridge and Hyde, said the killings had "brought shock and disbelief to the whole nation".
Witness Warren Shepherd was up a ladder cleaning the windows of a house nearby when the shooting happened.
He said: "I heard about 10 shots quite close... I knew they were gunshots because sometimes there is clay pigeon shooting in the fields.
"I heard a big explosion about 10 seconds after the shots. I got off the ladder, walked round, saw an empty police car.
"A car went speeding past to the main road."
He said he went back to his van but could see the aftermath through a gap between two houses.
He said the house where the incident took place was a council house which had been left empty.
He said there had been some recent activity with the windows covered in whitewash suggesting the house was being decorated.
The police flag at Greater Manchester Police headquarters has been lowered to half mast.
The force has also opened a book of condolence on its website.
Association of Chief Police Officers president Sir Hugh Orde said the deaths were "deeply sad news for the police service".
ACC Shewan said Mr Cregan had been arrested on suspicion of two counts of murder in relation to the investigations into the deaths of David Short and Mark Short.
David Short was killed in a gun and grenade attack in Clayton on 10 August.
His son, Mark, was killed in a pub shooting at the Cotton Tree Pub in Droylsden in May.
A second man - a 24-year-old - was also charged last month with the murder of David Short and later remanded in custody after he appeared at Manchester Crown Court.