The Cap Blanc, seen docked in St-Pierre-Miquelon in 2004, sank while carrying a large load of road salt it picked up in Newfoundland. (CBC)

At least one of four sailors was likely alive and trapped underneath a capsized French vessel when an RCMP vessel arrived on scene off Newfoundland, a senior French investigator said Tuesday.

But Jean-Pierre Mannic said it was too dangerous to send divers into the ship before it sank.

The French freighter Cap Blanc was loaded with road salt from Argentia, N.L., when it capsized en route to its home port in St-Pierre-Miquelon, on Dec. 2, 2008.

All four crew were lost, prompting an inquiry by the Paris-based Office of Inquiries and Analysis of Sea Accidents with assistance from Canada's Transportation Safety Board.

Mannic, the director of the French agency, told the Canadian Press that at least one member of the crew, and possibly two or three sailors, was alive and caught in a pocket of air under the hull many hours after the capsizing.

He bases his conclusion on testimony from officers aboard the RCMP patrol vessel Murray, which was the first rescue vessel to approach the capsized hull.

"I think they had been alive for 17 hours … because there were some frantic knocks on the hull. There was a response. It's a fact," Mannic said in a telephone interview from his office in France.

"One person, maybe two or three," were alive, he added.

There had been previous reports from search and rescue officials about the sounds, but Mannic's statement is the first confirmation by investigators.

Mannic also said an emergency signal on the vessel, known as an EPIRB, was turned on by one of the sailors, but that it was trapped under the ship and never transmitted its signal.

He said that was "very unfortunate," because if the device had reached the surface it could have set off a search for the Cap Blanc several hours earlier.

Search and rescue officials have confirmed they waited about 10 hours before starting a full-scale air search after the coast guard repeatedly tried to make radio contact with the vessel.

Had the emergency beacon surfaced, it would have caused a search to begin almost immediately, said Mannic.

Sgt. Wayne Newell, a spokesman for the RCMP in Newfoundland and Labrador, confirmed that the patrol vessel's crew believed people were making the knocking sounds.

"The crew of the Murray upon arriving on scene did notice some knocking sounds," he said. "Certainly their perception was that there were signs of life and they were making calls for help."

Both Newell and Mannic said the Murray wasn't equipped to provide divers to search the vessel, and the rescue co-ordination centre also determined it was too dangerous to attempt a rescue dive.

The French government commissioned a deep-sea submarine to search the wreck and recover the bodies of three crewmen who were aboard. The fourth man was not found.

The bodies of Jean Guy Urdanabia, Thierry Duruty and Robert Marcil were recovered, while the body of Robert Bechet remains missing.

Mannic said the final report on the cause of the sinking will be out before the end of March.

Safety experts have said proper loading of a small roll-on, roll-off vessel is crucial to its stability, especially in rough weather.

Mannic has said that loading is one of the areas the investigation examined.