The extensive discharge in the Bothnian Sea last week is being investigated as an environmental crime.
The discharge took place far out to sea and extended north of Gävle and up to Härnösand.
And now there are more and more indications that it was tall oil that was released - a viscous liquid that is used, among other things, to produce biofuel.
Even if the liquid comes from nature, it can cause damage to both animals and nature if it reaches land, says Ingrid Håstad, oil decontamination expert at Sweco.
- It contains substances that can create allergic reactions if you get it on the skin.
Dogs can get sick, birds can get it in the feathers, she says.