A new investigation has begun into what caused the sinking of the Baltic Sea passenger ferry M/S Estonia, which sank in 1994, claiming 852 lives.
The ship was travelling from the Estonian capital Tallinn to Stockholm on September 28, 1994 when it went down in the middle of the night off the coast of Finland.
Of the 989 people aboard, only 137 survived. It is considered Europe's worst maritime disaster since World War II.
Ships from Sweden and Estonia reached the site of the accident on Friday and began preliminary surveys, in research that is due to continue until 18 July.
The Swedish Accident Investigation Commission said the Swedish ship Electra af Asko and the Estonian vessel Eva 316 were at the site.
Bishops from Finland, Sweden, Estonia and Latvia held a memorial service on a vessel belonging to the Estonian border guard before the underwater work began.
Researchers used a multi-beam echo sounder in work that continued until evening, when they broke off due to the high seas.
The wreck lies at a depth of almost 80 metres and Saturday's work was dependant on weather conditions.
Sonar devices are to examine the ship and the seabed and a robot fitted with a camera will aid the probe.
Researchers at Stockholm's SU University are supporting the data gathering operation. The images will be viewed later, in a process that could take several months.
A grave site
More extensive investigations are then planned for next spring.
Neither the dead nor the wreckage were ever recovered and an international agreement declared the ruins a grave site in 1995, banning any disturbances.
A 1997 inquiry found that the ferry sank after its bow door was torn off in a storm.
But to this day there are doubts about the cause of the accident, and survivors and bereaved relatives have long demanded a fresh investigation.
The 1995 law that banned activity at the grave site was amended to allow further investigation into the shipwreck, with the change entering force in early July.
The new probe is being coordinated by Sweden, Estonia and Finland.