Sunday marks exactly six months since a fatal sightseeing boat accident occurred off the Japanese northern island of Hokkaido.

The Japan Coast Guard is still working to find out why the "Kazu I" tour boat with 26 people on board sank off Shiretoko Peninsula in April. Twenty people have been confirmed dead and six others are still unaccounted for.

The investigation is expected to take longer due to the lack of any eyewitnesses. Sources say a GPS-based position locater onboard the vessel is difficult to analyze because of damage sustained when the ship sank.

The Japan Coast Guard says it is working with experts to examine the salvaged ship. They are also restoring data from onboard electronics devices.

Recovery of such data can help identify how fast the ship was going and where it was sailing before it sank.

Coast guard officials say they are looking at all possible causes as there were no eyewitnesses.