Police investigating links between the murders of two officers and a criminal feud have declared: "Enough is enough."
The killings of Pcs Nicola Hughes, 23, and Fiona Bone, 32, in a gun and grenade attack in Hattersley, Tameside, on Tuesday showed "this has to end", assistant chief constable Garry Shewan said.
His comments came as a 22-year-old man was arrested after a Facebook page was set up lauding suspected police double killer Dale Cregan as a "legend". The country's top prosecutor, Keir Starmer QC, earlier said "the time has come for an informed debate about the boundaries of free speech in an age of social media".
A warrant of further detention was obtained on Wednesday night for Cregan which will expire at 9am on Friday. A 28-year-old man who was arrested on Wednesday in the Hattersley area on suspicion of conspiracy to commit murder also remains in police custody.
Tuesday's murders were a "watershed moment", Mr Shewan said, adding that the message from Greater Manchester Police to the two feuding families was: "Enough is enough. This has to end."
Mr Shewan said: "I think what we've seen on Tuesday has had to appeal to everyone in terms of a watershed moment. I would say the strong message that Greater Manchester Police has been using over the last six weeks, and perhaps even further back, is, 'Enough is enough. This has to end'.
"That's why we've been working with individuals with threats-to-life notices as part of our investigation. That's why we've had such an intensive policing operation in place. And that's why Tuesday's events have hit us so hard."
He went on: "We were investigating a feud between two criminal families, we continue to do that, and the link between that and Tuesday's events is still part of an ongoing and active investigation."
As tens of thousands of tributes poured in from across the nation for the fallen officers, more than one Facebook page was set up in praise of the suspect held by police. One was titled Dale Cregan OBE, with internet trolls posting messages claiming the alleged killer was a "hero".
The 22-year-old man, from Netherley, Merseyside, was arrested on Wednesday night on suspicion of committing an offence under the Communications Act 2003, Mr Shewan said.