The 96-metre (315-foot) Caffa was headed for Saint Petersburg, Russia, when armed Swedish police boarded it on Friday off the southern town of Trelleborg.
Sweden's coast guard said the ship was on Ukraine's sanctions list and was sailing under a false Guinean flag.
Russia's embassy in Stockholm has said that 10 of the 11 crew members are Russian nationals.
"Findings that have been made have led to a crew member being detained and brought ashore," the coast guard said in a statement on Sunday.
"The person is suspected of violations of the maritime code and the ship safety act, as well as the use of a forged document, which is an aggravated crime," it said.
The prosecution authority has taken over the preliminary investigation, which is continuing its investigation on board the ship, the coast guard said.
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Moscow's 'shadow fleet' consists of vessels used to skirt Western sanctions. They are often ageing ships in poor condition, without proper insurance and with opaque ownership.
The coast guard said it believes the Caffa has "extensive deficiencies" linked to its operation and activities.
"Our mandate is to uphold the regulations at sea. We will take action against vessels that violate them and that jeopardise safety in our maritime territory," the coast guard's acting head of operations, Daniel Stenling, said in a statement.
The Swedish Transport Agency was to inspect the ship and determine whether it was seaworthy and authorised to continue its journey.
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