Greater Manchester Police has warned of the threat from a long-standing feud between two criminal families in the area and has issued Osman warnings - notices given to people under threat of being murdered or seriously injured - to "a large number of individuals".
Assistant Chief Constable, Garry Shewan, said the murders of PC Fiona Bone and PC Nicola Hughes were a "watershed" moment and that the force's message was "Enough is enough. This has to end."
He added that despite everything the force has done to tackle gun crime, "tragedies still do occur".
"Tuesday's events simply make us spur on to higher and greater efforts."