THE officer leading the investigation into the death of accountant Andrew Ramsay has revealed for the first time that his murder may have been linked to a UK-wide Customs and Excise inquiry into a multi-million-pound "carousel" fraud.

Ramsay was interviewed by customs officials five times in the weeks before his disappearance. Police investigating his death have liaised with their counterparts, who are still carrying out the massive investigation into "carousel fraud".

The scam involves VAT registration being obtained for computer chips and mobile phones through a VAT-free loophole abroad. Goods are brought to the UK, resold at VAT-inclusive prices, leaving the fraudsters to pocket the profits, and then disappear without paying the tax.

Two years ago on Friday, the 51-year-old father-of-two was "arrested" by two bogus police officers on a busy street in the Cardonald area of Glasgow and bundled into the back of a black Honda Accord. Police have released an image of a W-registration Accord in an attempt to encourage the public to provide potentially vital information.

Police believe Ramsay was probably murdered and dumped by boat in the Firth of Clyde after his abductors found out he was questioned over the fraud.

Officers suspect he may have been killed as a result of what he may have told his abductors or their accomplices about what he had revealed to the inquiry. Ramsay had not been arrested or charged in connection with the Customs inquiry.

Detective chief inspector John McGovern, who is leading the murder investigation, launched a new appeal for information, and revealed fresh details of the complex inquiry, which has yielded only a handful of leads, despite the discovery of Ramsay's skull near the island of Little Cumbrae in Ayrshire almost 14 months after his disappearance on February 22, 2006.

McGovern also disclosed that sophisticated underwater search equipment has been used to comb the seabed off Little Cumbrae.

Police consulted Dr Justin Dix, a marine geophycisist from the National Oceanography Centre in Southampton, experts from the Royal Navy and military, and pored over tidal and wave charts of the area, which reaches a depth of 115 metres, one of the deepest parts of the firth.

Sonar equipment was deployed to check for unidentified objects on the seabed. Each has been eliminated, but the search is set to resume shortly after being halted due to bad weather.

Because the seabed is more than 60m deep, divers cannot take part in the search and Strathclyde Police leased an unmanned and remotely operated submersible, which is lowered to the seabed to photograph objects identified by a sonar device on a surface vessel. A Strathclyde Police underwater unit was assisted by Cheshire Police's dive unit, which owns some of the equipment.

It is similar to the search three years ago in the firth for Allison McGarrigle, 40, a mother-of-three from Rothesay, who vanished in 1997. She had been due to give evidence against a paedophile ring. Her body has never been found.

McGovern added: "This is a complex investigation, and is the first time we have been involved with the level of sophistication needed to search these depths.

"We have never tried technology like this before. It's a laborious process not helped by the often horrendous weather conditions."

McGovern is exploring the possibility that Ramsay's body may have been protected by material or weighed down to prevent it washing ashore; another sign of the extensive preparations behind the killing. Forensic tests have yet to establish the cause of his death.

Twice-married Ramsay set up Genk Electronics, in Belgium, where he lived for more than a year until he returned to Glasgow in May 2005. After his death, Customs officials travelled to Belgium to interview one of his friends about the company.

Back in Scotland, Ramsay did some accountancy work for local taxi firms and local businesses. His local bar was the Quo Vadis, and he was a keen hillwalker.

McGovern said a number of vital questions needed answered: did the interviews by the customs officials result in his death? Who were the bogus "cops"? And what, if any, was their role in his death?

He added: "Was he also taken somewhere after being abducted, to speak to somebody else about the information he had given?"

McGovern said that the answers could provide the crucial pieces in the complicated "jigsaw puzzle".

He said Ramsay may have been in a "no win" situation with his abductors if he was questioned about his involvement in the "carousel" fraud investigation. He added: "There is nothing else contained in our investigation to suggest any other reason than the VAT investigation that somebody would have an issue with Andrew Ramsay. This kind of fraud is organised crime.

"There was a 13-and-a-half-month gap when we didn't have a single sighting, his bank accounts weren't touched, and he wasn't in contact with anybody, before his skull was found.

"I believe that Andrew Ramsay is someone who got involved with the wrong people and that may have resulted in his abduction and death."

Ramsay's family broke their silence to issue a statement to the Sunday Herald last week. It read: "We are fully supportive of the police and the progress of this investigation. We would ask people to come forward and speak to the police if they know anything about what happened to Andrew, in order that we can move on with our lives."

Anyone with information is asked to contact the incident room on 0141 532 4767/4769, Strathclyde Police contact centre on 0141 532 2000; or Crimestoppers on 0800 555111 It's really difficult, like looking for a needle in a haystack' MARINE geophycisist Dr Justin Dix helped Strathclyde Police decide whether to carry out the extensive search by poring over data on wave and tidal flows in the area where Andrew Ramsay's skull was found.

The specialist in sonar and geological techniques at the National Oceanographic Centre, co-run by Southampton University, was contacted by the National Policing Improvement Agency, which advises UK forces on investigation support.

Dix also examined factors such as wave-induced flows, sediment movement and long-shore drift before concluding the accountant's body would not have travelled far if dumped south of Little Cumbrae.

Last year's operation used two high-resolution sonar devices to pinpoint and identify small objects that showed up as "black blobs" on the surveys.

The team, operating from a launch, used a side-scan sonar to take a wide area survey to both understand the "dynamics" of the bed and identify potential targets that could possibly be a body. A second seabed-mounted sonar was deployed to scan the area around a "target" and to guide an ROV submersible to take photographs. Dix said: "Everything suggested there wasn't a huge amount of sediment movement, making it unlikely a body would have moved far.

"It's a really difficult case, but I found that the flat sea bed in that area meant it was worth continuing because targets' would stand out sufficiently well on the sonar. It is like looking for a needle in a haystack."

He said more work could still be done to help police working on difficult underwater cases. "I am interested in trying to develop the combination of skills and expertise to help the police work in challenging marine environments."