Canoe Forest Products says it is investigating what caused the sinking of one of its tug boats on Shuswap Lake.

The 26-foot vessel was carrying approximately 4,000 litres of diesel fuel when it sank off of Ruckell Point on Monday.

A sheen of diesel was visible on the surface of the lake shortly afterwards, prompting a water advisory for residents of St. Ives, a community 10 kilometres away.

A dive crew has been deployed to plug off a vent thought to be the source of the leak.

Canoe mill general manager Marcello Angelozzi said the company's internal investigation has not yet determined the cause of the sinking, but he suspects the tug may have become hooked on a concrete anchor block.

The vessel's captain was picked up by a second tug boat and was unharmed.

Angelozzi said his team is determining a recovery plan for the vessel, which is now 90 feet below the lake's surface.

"We hope that we can throw a crane out on a barge and we can get the tug out of the water safely without spilling additional fuel. We're not sure exactly how much has been released at this point," he said. 

Angelozzi said they won't be able to determine how much diesel was spilled until the vessel is recovered.

For the moment, the surrounding area has been cordoned off with absorbent boom material.