A second man has been arrested in connection with the deaths of two unarmed police officers in a grenade and gun attack.
Greater Manchester Chief Constable Sir Peter Fahy said a 28-year-old man was detained in Hattersley on suspicion of conspiracy to murder.
Dale Cregan, 29, is also being questioned over the officers' deaths and the killings of father and son David and Mark Short.
The home secretary, Theresa May, flew into Manchester earlier on Wednesday as the family of one of the two unarmed police officers killed in an apparent ambush spoke of their loss.
May broke short her holiday to join Fahy in the city as the investigation into the double murder of the officers Nicola Hughes, 23, and Fiona Bone, 32, continued.
Hughes's parents Susan and Bryn, and her brother Sam, paid tribute to her. Speaking of their "beautiful child", her parents said: "Nicola was our only daughter. She was always happy with life and lived for her family.
"She had an infectious personality and sense of humour and was a very caring and loving girl. Nicola was only 23 years old and had her whole life ahead of her. We cannot express how we feel today except to say we have always been exceedingly proud of Nicola and always will be."
Bone's family were due to be met by members of Greater Manchester police after flying into Manchester from their home in the Isle of Man. A minute's silence was held by Greater Manchester officers at 11am, including by those at the scene of the attack. With heads bowed, colleagues of the dead women stood behind the police cordon. One could be seen wiping away tears.
Fahy was forced to defend the decision to release Dale Cregan, the first suspect arrested over the killing of the police officers, on bail in June.
The police have confirmed that Cregan, was arrested in Manchester airport on 12 June on suspicion of the murder of Mark Short, 23, in Droylsden, Greater Manchester, in May. But he was released on police bail after questioning.
Two months later on 10 August Short's father, David, 46, was murdered in a grenade and gun attack at his home.
The killing sparked a five-week manhunt involving 50 armed raids and a £50,000 reward. It only ended when Cregan handed himself in on Tuesday, following the fatal shooting of the two officers.
Fahy said: "It is absolutely normal in the course of complex crime inquiries that when people are arrested there are occasions where there is insufficient evidence available for them to be charged.
"In those circumstances suspects have to be released on bail as there are strict time limits covering how long suspects can be held in custody without charge. That is exactly what happened in this case."
Police believe the officers were lured to their deaths on Tuesday morning by a false report of a burglary at a property in Abbey Gardens on the Hattersley estate in Mottram, Greater Manchester.
As they arrived at the property the officers came under fire from bullets and a grenade.
Cregan, who walked into a police station in Hyde an hour later, is being questioned on suspicion of the murders of the two police officers and the father and son.
As the police investigated what they suspected was a "criminal conspiracy" to shelter Cregan, Tony Lloyd, Central Manchester MP and Labour candidate for the newly created post of police commissioner, said: "Those who did know the whereabouts of Cregan will be asking themselves why they did not make that sort of information available to the police. It is as simple as that."
The worst police killings since three officers were shot in west London in 1966 prompted calls for the death penalty to be restored. "Bring back hanging," Bone's father told the Daily Telegraph. "Let policemen shoot people on sight. I am just so completely shocked. She was such a lovely girl. I can't even bring myself to think about the wedding."
Lord Tebbit, the former Conservative party chairman, also said in the Telegraph it was time to think again of the "deterrent effect" of a death sentence. "I have kept track year by year since the death penalty was suspended then abolished of the number of people who have been killed by persons previously convicted of homicide," he writes. "It has averaged three people a year. About 150 people killed because their killers have been freed to kill again. I think it is time we thought again about the deterrent effect of the shadow of the gallows."