REYKJAVIK, Iceland (AP) _ A maritime court has begun hearing testimony in an effort to determine why a cargo ship crashed into a rock marked by a lighthouse and sank, killing all 12 men aboard.

The 2,600-ton Syneta, owned by a British company and registered in Gibraltar, sank in good weather Christmas Day after hitting the 531-foot-high Skrudur rock near the fishing port of Eskifjordur on Iceland's east coast.

''There are several possibilities as to what caused the tanker to sail straight into Skrudur rock,'' said Bjarni Stefansson, deputy to the sheriff for the eastern fjords of Iceland. He said authorities had not yet determined the cause.

The maritime court opened its hearing Sunday night in Eskifjordur. Sigurdur Eiriksson, sheriff of the eastern fjords, presided over the session. The ship's owners and operators and insurance representatives attended.

Stefansson said police completed their investigation Sunday. He said they questioned those involved in the rescue operation, including the crews of fishing boats and trawlers based in east coast fishing towns.

''The material recovered in the investigation will be used as evidence in the court but I cannot comment on the findings or give any substance,'' Stefansson told The Associated Press.

Johannes Briem, the chief rescue coordinator for the Icelandic Lifesaving Association, has said the Syneta's captain indicated in radio calls to the shore that the ship's radar system malfunctioned.

A letter found on the body of one crew member complained of engine problems and of an inoperable automatic pilot, Icelandic officials reported.

The British ship officers union said it would press the British government for a separate inquiry because it was unhappy with Gibraltar's safety standards for vessels registered there.

''We have no input whatsoever into the Icelandic inquiry, so we cannot ask questions,'' said Eric Nevins, general secretary of Britain's 26,300-member National Union of Marine, Aviation and Shipping Transport Officers.

John Prescott of Britain's opposition Labor Party said the government should urge Gibraltar to hold an inquiry.

''The evidence we have of flags of convenience countries is that they sell their flags for a few pieces of silver,'' Prescott said.

But a Department of Transportation official said ''calls for an inquiry are a matter for the Gibraltar authorities'' because the Syneta was registered there.

The Syneta was headed for Eskifjordur to pick up a cargo of fish liver oil when it sank. The crew consisted of Capt. Richard Cape, a Briton, five other British officers and six seamen from the Cape Verde Islands on the west coast of Africa.

Crews continued to search Sunday for the bodies of three crew members still unaccounted for, but were hampered by bad weather. The missing are presumed dead.

A day before the sinking of the Syneta, the Icelandic freighter Sudurland sank in rough seas between Iceland and Norway, killing six people. The five survivors said in Reykjavik on Sunday they owed their lives to a British Royal Air Force reconnaissance plane that dropped a new lifeboat to them as they huddled in a leaking dinghy.