A coroner leading an inquest into the deaths of two French fishermen 15 years ago says he wants to "dispel speculation and conspiracy theories".
Yves Gloaguen and Pascal le Floch died when the Bugaled Breizh sank off the Lizard, in Cornwall, in 2004.
An inquest in Truro has resumed two years after a French judicial investigation was completed.
It is scheduled to last nine days, with evidence to be heard from military personnel involved with the rescue.
Relatives of the crew have said they believed the trawler was dragged underwater, 14 miles off the Lizard Peninsula, when a submarine became snagged in its nets.
There were British and Dutch submarines in the area for Nato military exercises.
The inquest was told a submarine joined in a search after the Bugaled Breizh sent out a mayday message.
Serge Cossec, skipper of the French trawler Eridan, which received the mayday, was the first on the scene.
Other vessels joined in the search, including a submarine, he said.
His statement, given to police and based on his sea logs, added: "I saw a submarine on the surface moving slowly. I tried to approach it but it seemed to be evading approach... I couldn't identify it."
However, France's top judicial court in 2016 discounted any submarine involvement in the sinking, despite speculation. The UK's Ministry of Defence has always denied involvement.
Acting Senior Cornwall Coroner Andrew Cox told the hearing he had read thousands of pages of evidence ahead of the inquest resuming.
He said: "There are difficulties in conducting an inquest 15 years after the event, with a different language involved and when some witnesses are not here."
No members of the bereaved families attended the first day of the hearing on Monday, but the Ministry of Defence is represented.
The inquest is being held in Truro because the bodies of Mr Le Floch and Mr Gloaguen were taken to the UK after they were found by the rescue helicopter. The trawler's three other crew also died.
The body of one of those men was brought back to France and the two others have never been found.
The inquest again heard the official cause of death for both the fishermen was drowning.
The hearing continues.