Cold case detectives re-investigating the shooting of a banker 20 years ago say they are keeping an open mind to the murderer’s motive.

Dad-of-two Alistair Wilson was shot on the doorstep of his family home in the Highland seaside town of Nairn on 28 November 2004. He later died in hospital.

The murder remains unsolved and Mr Wilson’s family have criticised police efforts to find the killer.

Officers previously said they had a potential motive and suspect, but the senior detective overseeing the new probe said there were many open lines of investigation and nothing was being discounted.

Mr Wilson, 30, and his wife Veronica had been bathing their two young sons and getting ready to read them a bedtime story when the killer came to the door.

A blue envelope, which had the name Paul on it, was handed to Mr Wilson on the doorstep.

He went inside and showed it to his wife, but the envelope had nothing inside.

When Mr Wilson returned to the door he was shot.

Last year, police said they had a potential suspect after previously suggesting a planning dispute over decking at hotel near the Wilsons’ home was a likely motive.

But on the 20th anniversary of the murder, Det Ch Supt Suzanne Chow vowed the re-investigation would be “full and thorough”.

She told BBC Scotland News: “We are not discounting any avenue at this time and there a number of lines of investigation open to us.

“We are taking it right back to what happened in 2004.

“We are being totally impartial, totally open minded and ensuring a thorough investigation is carried out.”

The senior detective said she had met Mr Wilson’s family to “build trust and confidence” with them.

And she said a team of officers were collating and evaluating 20 years of evidence.

The cold case probe will also take advantage of new techniques and technologies that were not available in 2004.

Officers who have dealt with similar types of cases with police forces in England, Wales and internationally are also being consulted.

Police Scotland has set up a major investigation public portal where people can submit files and information anonymously.

Det Ch Supt Chow, who has been involved in other cold cases including the 1984 murder of Mary McLaughlin, said the re-investigation would take as long as was needed.

She added: “It is a really complex investigation and I am committed and determined to finding answers for Alistair’s family.”

The re-investigation, called Operation Sabine, was ordered by Scotland’s top law officer in September.

Since the murder, police have released various pieces of information on the killer:

• He was aged between 20 to 40 years old

• He was about 5ft 7in and wearing a baseball cap and jacket

• The gun was a Haenal Suhl Model 1 Schmeisser’s patent handgun, manufactured between 1920 and 1945 at the Schmeisser factory in Germany.

• The ammunition was .25 calibre made by Sellier and Bellot in the Czech Republic between 1983 and 1993.

In December last year, Mr Wilson’s family complained to the Police Investigation and Review Commissioner (Pirc) about the handling of the force’s investigation.

Police Scotland and the Crown Office said at the time they had met the family to discuss the matters raised by them.

Then in September this year, the family said they had “lost confidence” in the national force and labelled it “incompetent”.

Mrs Wilson and her son Andrew, who is now 24, told the Press and Journal newspaper that their relationship with police had “steadily deteriorated” in the past 18 months.

The family said this was due to unresolved issues not being properly addressed.