A criminology professor says he has been sent information that could provide new clues to the shooting of a man in November 2004.
The case remains unsolved, but police continue to actively investigate the murder father-of-two Alistair Wilson, 30, at his home in Nairn.
Prof David Wilson said the new details included a claim Mr Wilson's wife was not the sole witness to the shooting.
The information has been passed to Police Scotland.
A police spokesman said the case remained active and ongoing and the force was "limited" on what it could say on the investigation.
Prof Wilson, Professor of Criminology at Birmingham City University, told BBC Radio Scotland's John Beattie programme he believed the case was now "eminently solvable".
He said the information was sent to him a package from a person identified only as "Nate".
The sender of the package claimed there was an independent witness to the murder on the doorstep of the Wilsons' home and named this person.
Further details of this other witness were not revealed on air, but have been passed to the police.
Previously, Prof Wilson said Mr Wilson's widow Veronica was believed to have been the sole witness.
A second piece of information in the package was a claim that the name on an envelope handed to the Mr Wilson before he was shot was "Paul".
Prof Wilson said: "I'd always previously assumed that this was a master hitman who carried out this particular crime and the case would be unsolvable.
"Because of what the package contains, if it is accurate, I now think it is eminently solvable."
He questioned, if there was an independent witness, why the police had not produced an e-fit image of the killer.
The professor also said he wanted to know if the police had a forensic accountant examine Mr Wilson's banking clients.
The academic said he initially believed the eight-page package, entitled Alistair Wilson: A Cold Case Thesis, which was sent to his university address with what he believes is a Glasgow postmark, came from a "good armchair detective".
However, the new information contained in the document made him reconsider.
Alistair Wilson: The police investigation
Officers from former Northern Constabulary, and now its successor force Police Scotland, have committed many hours attempting to solve the case.
By 2015, detectives had interviewed about 2,700 people in their efforts to find the killer, a stocky man in a baseball cap, and taken more than 3,500 statements.
One of the biggest breaks in the case came days after the murder with the discovery of the weapon used in the shooting.
The gun involved was found on 8 December 2004 in a drain on Seabank Road, Nairn, by council workers carrying out gully cleaning.
Forensic analysis identified it as the murder weapon.
Detectives later established that the Haenel Suhl Model 1 Schmeisser was manufactured between 1920 and 1945 at a Schmeisser factory in Germany.
The ammunition used was .25 calibre made by Sellier and Bellot in the Czech Republic between 1983 and 1993.
About 40,000 of the guns were produced in Germany and police suspect the weapon may have been brought back to the UK either as a war trophy, for legitimate export or on the black market.
Mr Wilson, 30, was shot at his home at about 19:00 on 28 November 2004. He later died in hospital.
Mr Wilson's killer - a stocky man with a baseball cap - handed him an envelope before shooting him with a German-made handgun.
A massive police inquiry was launched at the time and detectives continue to investigate the case.
Det Supt Gary Cunningham, of Police Scotland, said: "The investigation into the murder of Alistair Wilson remains open and ongoing and we urge anyone with new information or anyone who hasn't spoken to police to speak to us so we can investigate all opportunities.
"As the investigation is live, we are limited in what we can discuss, however, we can confirm that investigations into Alistair's personal and professional life have been a focus on the inquiry into his death. These matters remain under review.
"A significant amount of work was done to identify all individuals who were in the vicinity of the murder and we have taken over 3,500 statements throughout our investigation."
He added: "There is limited detail available about the facial features of the suspect from witnesses and as such, there is not sufficient information to produce an e-fit."