Macedonia canceled all public events Sunday to mourn 15 people who died when a tourist boat sank in a southern Macedonian lake, local media reported.

All flags on Macedonia's public buildings will be flown at half-mast in memory of the dead. It has been confirmed that all victims are Bulgarian tourists.

Macedonian Prime Minister Nikola Gruevski sent a note of condolences to his Bulgarian counterpart Boyko Borisov and the victims' families.

"I was deeply shaken by the tragic accident in the Ohrid Lake and the death of the Bulgarian citizens. In these difficult moments of national mourning, I would like to extend my deepest condolences to you and your people on behalf of the Macedonian government," he said.

The accident happened when the tourist boat, called the Ilinden, sank around 11 a.m. (0900 GMT) on Saturday about 200 meters off the shore of the Ohrid Lake, a resort about 190 kilometers south of the capital Skopje.

The boat had been traveling from Ohrid, on the lake's eastern shore, to a medieval Eastern Orthodox monastery complex about 30 kilometers to the south near the Albanian border.

There were 57 people, mostly Bulgarian tourists, on board. Fifteen passengers drowned, while 42, including two crew members, survived the accident.

Earlier reports had put the number of passengers on-board at 73. Police later corrected the number.

The boat, with a capacity of 42, was built in Germany in the 1930s. It was examined and given a license to operate last May.

Initial investigation shows that a rupture of the cord that links the rudder with the stern caused the boat to turn sharply to the right side.

The passengers, by inertia, crossed to the other side. Filled with water, the boat leaned and sank in no time.

However, Macedonia has invited a team of three experts from Croatia, a regional ship-building power, to help investigate the cause of the accident.

Lake Ohrid, embedded on Macedonia-Albania border, is the deepest in the Balkans. It has been a UNESCO world heritage site since 1979.

Source: Xinhua