Chief Superintendent Steve Neill says officers would like to hear from people who may have seen anything unusual in Stanley Street at any time in the last few days.
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Callum Young, who will now be 23, was last seen on August 27 last year at his home in Hylton Road.
Gunner Paul Ferguson was on leave from the Army when he died in the early hours of Sunday August 21.
Dutch International Darryl Janmaat left for Watford, USA's DeAndre Yedlin arrived from Spurs and Tim Krul left for a season's loan at Ajax.