BARCELONA, Spain (AP) — Spanish state prosecutors have opened an investigation of countless tiny plastic pellets washing up on the country’s northwest coastline after they were spilled from a transport ship, posing a possible major ecological problem in the area.

The state prosecutor’s office made the announcement late on Monday after having studied the arrival of pellets on the shore during the previous weeks.

The prosecutors fear that the pellets could have toxic properties and added that there are indications that they have also been found on Portuguese and even French shores.

The spill was first reported to authorities on December 13 when hundreds of thousands of tiny white balls began washing up on Spain’s Atlantic shoreline.

Spain’s government representative for the northwest Galicia region said that the container ship Toconao, sailing under a Liberian flag, lost six shipping containers off the coast of Portugal, some 80 kilometres (50 miles) to the west of Viana do Castelo.

One of the six containers contained 1,000 sacks of pellets, with each sack holding 25 kilograms of the tiny plastic balls used in the fabrication of plastic products, the government representative said.

Greenpeace and other environmental groups calculate the total amount of pellets lost to be in the millions. They say that the pellets represent a danger for marine and human life since they can break down into even smaller microplastics that can be consumed by fish that are later caught by fishermen.

“The contamination of the oceans and ecosystems with plastics is one of the biggest problems faced by humanity,” Spain’s minister for the environment, Teresa Ribera, said. “So the spilling of such an important quantity of plastics requires close oversight and to determine if the transport company and shipping company exercised the proper precautions.”

Maersk, the shipping company contracted to transport the containers, told The Associated Press in an email that the containers were lost on December 8 in the deep sea during the voyage from Spain’s southern port of Algeciras to Rotterdam, Netherlands. It said that the Toconao is a charter vessel, not one of the Danish company’s fleet, and it works on the company’s route between Northern Europe and the Mediterranean.

“None of the six containers contained dangerous goods. One of them was loaded with bags with little plastic pellets for the production of food-grade packaging like water bottles,” Maersk said in the statement. “The vessel owners have appointed multiple cleanup specialists to support removing the pellets.”

Maersk said it is investigating the cause of the lost containers to “take necessary steps to minimise the risk of similar incidents occurring in the future.”

Volunteers and workers have organized to clean up the beaches and coasts of the area, which depends on a large fish and shellfish industry. Galicia’s marine coastline was devastated by an oil spill from the Prestige tanker in 2002.

The EU’s executive arm, the European Commission, proposed measures last October to help prevent the mishandling and spillage of plastic pellets. The measures must be debated by the 27 EU member states and the European Parliament, and would enter force 18 months after any agreement is reached.

Jordi Oliva, co-founder of Good Karma Projects, a Spanish NGO dedicated to fighting microplastics in the sea, said that this is the largest single spill of pellets that his group has seen in Spanish waters. He hopes this incident can help push the EU and national authorities to act.

Oliva said the pellets act as sponges for toxins already in the water, turning them into toxic pills for any sea creature that eats them and entering the food chain that can lead up to human consumers.

“This adds to the problem of microplastics,” Oliva told The AP. “We must put the focus of the debate not on who cleans this up, because next month we could find ourselves running again (to clean another spill up) if there is no regulation to guarantee that this type of material is handled with care.”