The Swedish Coast Guard said on Friday it had released an oil tanker believed to be part of a Russian "shadow fleet" that it suspects is the source of a 12 kilometres oil spill off the island of Gotland.
The tanker, which was seized in the Baltic Sea, is believed to be part of a fleet of "shadow tankers" that Moscow has been using to fund its four-year war against Ukraine.
European nations, including Sweden, have increased efforts to disrupt the fleet, moves that Russia has condemned as hostile.
The tanker, Flora 1, was seized off Sweden's southern coast, the Swedish Coast Guard said. It had embarked from the Russian port Primorsk with an unclear destination, according to MarineTraffic data. The oil spill is not expected to reach shore, the coast guard said.
The Swedish Prosecution Authority said it interrogated the crew, and that two people had been informed of a suspicion of environmental crimes. The persons had not been detained.
"The spill occurred within Sweden's economic zone, which means that the investigative measures we can take are limited," the Prosecution Authority said.
"We have conducted the interviews that were deemed necessary and we will not take any coercive measures due to this suspected crime."
The authority said it had started an investigation regarding a suspected environmental crime. It said it understood the vessel was on the EU sanctions list, adding that its flag status was unclear.
"The Russian shadow fleet, consisting of older, poorly insured tankers that evade sanctions, poses a significant security and environmental threat," Swedish Minister for Civil Defence Carl-Oskar Bohlin said on social media, naming the Flora 1 in his comments.
"The government is taking the incident seriously, even though this time it is not a major oil spill."
(Reporting by Johan Ahlander; Editing by Alison Williams and David Gaffen)