French investigators have ended a probe into the mysterious sinking of a trawler off the British coast that relatives of five dead fishermen believe may have been caused by a NATO submarine.

A lawyer for the families of the victims, Christian Bergot, told AFP Thursday that his clients had been notified of the end of the investigation, but that he intended to request further inquiries into whether a US submarine may have been involved.

The Bugaled Breizh sank suddenly off Lizard Point in the English Channel on January 15, 2004, killing its five French crewmen.

Relatives of the victims have long suspected a submarine taking part in NATO naval exercises in the area could have been caught up in the trawler's cables and dragged it down.

The French Marine Accident Bureau concluded that the sinking was most likely caused by a cable snagging on the sea floor, but the investigation continued after traces of titanium, often used in the hulls of submarines, were found on its cables.

French authorities in January finally ruled out the theory, when two expert reports said neither the Royal Navy's HMS Turbulent, nor any other submarine, was involved in the accident. Earlier theories had also suggested a Dutch submarine was involved.

But Bergot said he would make a last-ditch attempt to have the probe re-opened by asking investigators to look into the possibility a US submarine was involved.

He cited expert testimony that a US submarine may have been in the area and said investigators should demand more information from Washington. The US Navy has already denied it had a nuclear attack submarine in the area.

Bergot said the request to the US would likely be the final attempt to link the accident with a submarine.

"There is still a window of opportunity, in my opinion we should use it," he said.

Dominique Tricaud, another lawyer for the plaintiffs, said he would also be requesting the investigation be re-opened into the Turbulent's possible involvement, denouncing "10 years of government lies and judicial cowardice" in the case.