The far-right terrorist who killed Labour MP Jo Cox may have been handed the murder weapon by an accomplice, police have said.

Detectives are investigating how convicted murderer Thomas Mair, 53, came to be in possession of the bolt-action rifle used to shoot Cox outside her Batley and Spen constituency library in the market town of Birstall, West Yorkshire, on 16 June.

"How he, an antisocial loner with no previous criminal history, no criminal ring of individuals around him, how he came to be in possession of that gun is very much an active line of inquiry," Detective Superintendent Nick Wallen, who led the investigation into Cox's death, told the Daily Mail.

The .22 calibre firearm used to shoot the mother-of-two twice in the head and once in the chest was traced by police 14 miles from Mair's hometown of Birstall to the West Yorkshire town of Keighley, where it was reported stolen from the boot of an SUV.

The German-made rifle had most of its barrel and stock sawn-off when recovered from Mair, and measured just 12 inches in length. There was no forensic evidence the modifications were done at Mair's home, with Det Supt Wallen saying he believed they were carried out by a third party.

He said: ''When it was stolen it wasn't sawn-off as it was when it was used."

When asked if Mair had planned to kill anyone else, Det Supt Wallen said: "The only person who can really answer that is Thomas Mair and he hasn't spoken to us we don't know what his firm intentions were.

"[T]here was the matricide search, so read into that what you will. Was he on his way home when police detained him? The answer is not really. Where he was going is only something he can answer."

It comes after the National Crime Agency (NCA) last month warned extremists had been caught trying to buy guns on the UK black market to carry out terrorist attacks.

They urged the public to report illegally held firearms and lax security among legitimate gun owners.

Figures for police forces across the country show 884 firearms have been seized by law enforcement officers in the past year. The haul includes a Skorpion sub-machine pistol, an Uzi 9mm machine pistol, a Mac-10 machine pistol, an assault rifle and at least 9,000 rounds of ammunition.

Senior officers also admitted that 800 legally-owned guns – like the one stolen and then used by Mair – had gone missing from owners, leading to fears they may end up in the hands of violent extremists.

Mair was sentenced to life in prison on Wednesday (23 November) after being found guilty of shooting and stabbing Cox to death, and stabbing a pensioner who had tried to come to the MP's aid.

The Remain campaigner's death, which came just a week before the EU referendum, was said by the prosecution to be premeditated and politically motivated.

A police search of Mair's council house uncovered a stash of far-right material, including Nazi memorabilia, white supremacist books and press cuttings on right-wing Norwegian mass-killer Anders Breivik.

His interest in the far-right appeared to be slow-burning and can be traced back to the early 1990s, when he started to engage with white supremacist groups abroad.

Cox's husband, Brendan, said after Mair's conviction he felt "nothing but pity" for his wife's killer, calling her murder "an act of terrorism".