An analysis of samples of the slick has shown it is a uniform mix of fuel and oils, which indicates that it is oil product waste, according to the Baltic News Service.
On Wednesday, President Arnold Ruutel visited the scene and urged the authorities to better monitor passing sea traffic and punish the crews of polluting vessels.
"Ship traffic in the Baltic Sea is constantly increasing, so Estonia must apply necessary measures to protect the sea," the president said.
A long oil slick was first noticed off the coast of the Baltic state on Saturday.
Since then, rescuers and volunteers have been collecting oil-coated birds to clean them and gathering up dead birds to prevent them from being eaten by other animals.
Many of the birds are out at sea, on weak ice, making the work dangerous and difficult.
Ornithologists have said seabirds ranging from mute swans, different duck species, grebes and seagulls have been affected by the slick.