Thomas Mair stabbed and shot Mrs Cox on June 16, 2016 with a sawn-off shotgun outside a library in Birstall as the MP attended a surgery to meet with constituents.
Described as a “self-radicalised national socialist”, detectives found multiple items in his home connected to the extreme-right, including Nazi memorabilia and a copy of Mein Kampf.
Mair was sentenced to a whole life order at the Old Bailey in November of the same year, with a judge saying the attack had been committed to "advance the cause of violent white nationalism".
David Lowe, a senior research fellow in terrorism and security at Leeds Beckett University, told The Yorkshire Post he was “deeply imbued with the national socialism for years”.
But the question of how he acquired the Weihrauch .22 rifle - which is most commonly used for pest control - used in the attack of Ms Cox has never been solved, despite it now being more than four years since the murder.
Following his conviction, detectives at West Yorkshire Police’s Homicide and Major Enquiries Team (HMET) continued to investigate how he acquired the weapon, with enquiries finding it had been reported stolen from the registered owner’s 4x4 at a farm in Keighley in August 2015 - 10 months prior.
In a later documentary on the murder, Detective Chief Inspector Nick Wallen, who led the investigation into Mair, said he “would not rest” until the mystery had been solved.
The gun, the detective said, was "pulled to bits" by forensics in a bid to find DNA traces that did not belong to Mair, but to no avail.
A subsequent search of Mair’s phone had found no record of any calls made and only three text messages sent in as many years, which the detective said was not indicative of someone “immersed in criminality”.
More than four years on, West Yorkshire Police have confirmed that all avenues in the investigation had been “exhausted”, but that it would revisit the case pending new evidence.
Det Chief Insp Wallen has meanwhile retired from the force at the end of 2019.
A spokeswoman said: “A lengthy investigation was carried out to try and establish how Thomas Mair acquired this firearm. All lines of enquiry have for the time being been exhausted, but the investigation will be revisited if further information comes to light.”
The Yorkshire Post has carried out a lengthy investigation into how terrorism has changed. You can read the full piece here.