Five surfers who knew the sea "like the back of their hand" have died after a huge layer of foam in the water hampered efforts to rescue them.
The group ran into difficulties at the northern harbor head of the Scheveningen district of The Hague in the Netherlands on Monday evening.
Despite a large-scale rescue operation, only one member of the group could be saved, according to KNRM, the Royal Netherlands Sea Rescue Organization.
A statement issued online by the rescue service said its efforts were "complicated by the man-sized foam layer at sea and on the beach," while "strong winds and high waves also made it very difficult to provide relief from the harbor pier."
Police, firefighters, the coastguard, units from KNRM and other emergency workers were all involved in the rescue operation, in which a helicopter was used to try to blow away the foam and improve visibility.
Two of the bodies were recovered on Monday and another two the following morning, according to the KNRM post. A fifth body has yet to be recovered. The victims' identities have not yet been released.
Investigators are now looking into what caused the sea foam and its possible role in the tragedy.
Johan Remkes, mayor of The Hague, described the events as a "terrible tragedy" involving "young, physically fit, sporty people, who know the sea like the back of their hand."
In a statement published on the city's website, Remkes offered his condolences and said: "The heavy grief among people in the Scheveningen community is unfathomable. People here understand better than anybody else that 'The sea gives and the sea takes', but the way in which so many young lives ended abruptly and so many families and groups of friends have been affected is incredibly brutal."
The mayor, who witnessed the incident as it happened, said: "How is it possible that such experienced surfers were completely taken by surprise at a spot they knew so well? But also: how is it possible that a drama unfolded at one harbour inlet while at the other harbour inlet, not even 100 metres away, people were still surfing until late in the evening?"
Pledging to investigate the incident, he said: "Today we remember the victims. Tomorrow we will start trying to understand what happened."
Katja Philippart is a marine ecologist at the Royal Netherlands Institute for Sea Research. She told CNN that she is investigating whether algae density may have affected the level of foam in the water.
Philippart said that the levels of the algae are often blooming -- or high -- in April and May.
"At the end of a bloom, these colonies start to deteriorate, releasing the mucus into the water. Wind-driven waves that stir up the mucus-rich water result in the formation of foam. Depending on the wind direction, this foam can be blown to the coast where it then accumulates."
"They were robbed due to the exceptional circumstances and fate has struck: 5 of them tragically died," the post said.
"We express our sympathy to the relatives and everyone involved. In the surf community in Scheveningen and far beyond there is great defeat, we are in deep mourning for this loss."