A Dutch shipping company has apologised for a “very unfortunate accident” after three funeral urns filled with human ashes washed up on beaches in the Netherlands, sparking fevered speculation about how they got there.

RTL News said the urns, found over the past five days by a schoolboy, a fisherman and a woman respectively on Katwijk and Noordwijk beaches north of The Hague, all came from the same German crematorium.

The aluminium lids were stamped with the dates of birth, death and cremation of the deceased, and marked “For collection” from Greifswald crematorium in Mecklenburg-Vorpommern, northern Germany – nearly 470 miles (750km) away.

“We checked it out, because we thought it could have been a disguise for something else, drugs for example,” Maarten van Duijn, the 14-year-old who found one of the urns with his father, told the website KustNieuws.

“But it turned out really to be human ashes. So we started investigating – and the more we looked, the more bizarre the whole thing appeared.”

His father Leen told the Algemeen Dagblad newspaper: “In 30 years of beachcombing I thought I’d seen everything, but never a full urn. You can see an empty one going overboard if ashes are scattered at sea, but three full ones?”

The German news site Ostzee-Zeitung reported that it was a complete mystery how the urns had ended up in the North Sea. “Strict rules apply in Germany to the handling of deceased person’s remains,” the site said.

“With rare exceptions, it is forbidden to keep ashes in the garden or inside the home, so they are almost always given to the undertaker. Burial at sea is only permitted in special biodegradable urns, which these clearly were not.”

A spokesman for the Stralsund public prosecutor’s office, which covers the Greifswald area, told the paper the case was “highly unusual” and the prosecutor was considering whether a crime had been committed – such as disturbance of the dead – that warranted formal investigation.

But on Wednesday the Dutch shipping company Trip Scheepvaart of Scheveningen in The Hague said a planned mass marine funeral had gone horribly wrong when the box containing the three urns “slipped from an employee’s hands over the railing”.

A spokeswoman, Silvia Roos, told Germany’s DPA agency the company had now buried the contents of two of the three urns at sea and would soon do the same with the third. It was considering how to apologise to relatives for the “most uncomfortable” incident, she said.