THE gun used by Thomas Mair to murder MP Jo Cox had been stolen in Keighley almost a year before the attack, West Yorkshire Police has revealed. 

Mair used a German-made .22 calibre bolt-action rifle which had been chopped down to 12ins in length so it could be used with one hand.

The weapon was stolen from a car in Keighley but police still have no idea how it got into Mair's hands, or whether he cut it down himself.

He also used a modern copy of a Second World War commando knife with a 7in (18cm) blade to inflict 15 stab wounds, including one which pierced the mother-of-two's heart in the attack in Birstall in June.

Speaking after Mair was given a whole-life term today, Detective Superintendent Nick Wallen said the gun was stolen from the back of a sports utility vehicle in Keighley in August 2015, the ammunition most likely at the same time.

While Mair, who never legally owned a firearm or had a firearms licence, has not been ruled out as the thief, Mr Wallen said it was "highly unlikely" that such an "absolute loner" would have travelled to steal it.

The weapon, previously legally owned, was not sawn down when it was stolen but officers do not believe Mair modified it himself.

A forensic search of his home and gardens found "no evidence" he cut it down, and Mair had not searched online for the necessary expertise to modify it.

Mr Wallen said: "My suggestion is that Thomas Mair has not been responsible for cutting the weapon down.

"But how he - an anti-social loner with no previous history, no criminal ring or individuals around him - how he came to be in possession of that gun is very much an active line of inquiry and I would like any assistance I can get."

Mr Wallen said he did not know if Mair had fired the weapon before, but if he did he ran the risk of someone seeing him and calling the police.

Bloodstains which were a "billion-to-one" match to both Mrs Cox and Mair were found on the gun and dagger when they were tested after being found in his bag when he was arrested, the court heard.

Numerous witnesses told similar stories of hearing a "popping" noise as Mair fired the makeshift weapon.

And several also spoke of seeing the white supremacist loading fresh rounds into the gun's magazine between shots at the already badly hurt politician as she lay helpless on the ground.

The Weihrauch rifle's bolt mechanism also required re-cocking between each shot.

Jurors were shown a YouTube video that Mair watched on June 7 of an American man firing a similar .22 sawn-off rifle in a field, filmed from a head-cam. This was the same day he searched for Mrs Cox on Wikipedia and Google Images.

The gun he used had all but 1.6ins of its barrel removed to create a highly illegal weapon - but one that also made it less powerful.

This meant the two shots that hit Mrs Cox in the head did little serious damage. He also hit her in the chest.

In Mair's bloodied black sports bag were 25 rounds of ammunition, including 12 .22 "hollow-point" cartridges made by British maker Eley.

Also sometimes known as "dum dum" rounds, they are designed to expand and break up on impact to create more devastating wounds. Such rounds are banned from being used by the military under international law.

Ballistics expert Andre Horne told Mair's trial they are popular for hunting, saying: "The idea of that is to cause a greater wound size, especially when hunting, which would be considered a more humane way of disposing of animals."

He explained this was because a bullet staying together and causing a smaller wound might allow an animal to escape, prolonging its suffering before it died.

He added: "They are most commonly used for hunting vermin, squirrels, rabbits and other small animals."

On its website Eley boasts of the ammo's "excellent stopping power".

Mr Horne said the bullets could be legally owned in the UK with the correct firearms licence. The same would also be true of the Weihrauch rifle in its original condition, but it would be illegal in its cut-down form, he added.

Kerry Versfeld, of the National Ballistics Intelligence Service, examined the gun, two spent cartridges found at the scene and items recovered from the post-mortem examination.

Testing "conclusively" determined that two cases from the scene and one found in nearby John Nelson Close were all fired by the rifle, she said.

Addressing the knife, Mr Horne said it was a Fairbairn-Sykes "fighting dagger", a design first made in 1941 for British special forces and commando units, with a near 7ins blade.

But he added: "It was determined that it was a fake replica and not one that was produced for the military."