Speaking after Mair was sentenced to life in prison today, the senior detective in charge of the murder investigating, Det Supt Nick Wallen, revealed after the case that the .22 rimfire weapon had been stolen from a 4x4 utility vehicle in the West Yorkshire town in August 2015.
The West Yorkshire Police detective said: “How it got to Thomas Mair is still a line of enquiry for me and the team.”
He added: “How he, an anti-social loner with no previous criminal history, no criminal ring of individuals around him, came into possession of it is very much an active line of enquiry and I would like any assistance as to how he came by it.”
Officers believe the weapon likely came into Mair’s possession just weeks before he murdered his local MP.
Mr Wallen confirmed that the weapon had previously been legally owned and had been stolen from the boot of a vehicle.
The officer said he believed Mair began planning that attack after only after he got the rifle, around the time he made internet searches on how to kill someone with a .22 weapon.
Mr Wallen said Mair began to “immerse himself in hate”, as was evident from him accessing neo-Nazi and other far right material online.
He added that it was still unclear why Mair chose Cox as a target other weather he was planning a “spectacular” and killing others as he re-loaded the weapon and headed away from the centre of the village.
The officer added: “What we know about Thomas Mair is that he is a loner in the truest sense of the word. He had no mental health diagnosis but showed signs of obsessive compulsive disorder.”