The Finnish authorities said on Sunday that they had released an oil tanker seized in December over suspicions that it had deliberately cut vital undersea cables but that a criminal investigation into the episode would continue.

The authorities said last year that the ship, the Eagle S, appeared to belong to Russia’s shadow fleet — older tankers that covertly transport Russian crude oil around the world — escalating concerns about a covert campaign to sabotage European infrastructure.

On Sunday, the Finnish police said that since the criminal inquiry “has progressed,” the aging tanker was free to leave and that border officials had escorted the ship out of the country’s territorial waters.

Petteri Orpo, Finland’s prime minister, said in an interview with Yle, the country’s public broadcaster, that “the criminal process and investigation will continue.”

Investigators were still examining materials gathered after an onboard “forensic investigation” and would continue to interview the crew, according to the police.

Eight crew members are suspected of criminal offenses, including aggravated criminal mischief and aggravated interference with communications, the police said in a statement. Five were allowed to leave Finland last week, while the other three were still barred from leaving, according to the statement.

The police said the authorities hoped to conclude the investigation by the end of April.

The cutting of the cables under the Baltic Sea in late December came on the heels of a series of similar incidents and prompted NATO to bolster security in the region. In January, the Swedish authorities also seized a ship and said they suspected “gross sabotage” after a different undersea cable was damaged. Last month, the European Union vowed to increase security after another cable break.

The Eagle S, registered in the Cook Islands in the South Pacific, had been sailing from St. Petersburg, Russia, to Port Said, Egypt, when it was seized.

Western officials have long feared that Moscow’s so-called shadow fleet could be used to circumvent sanctions imposed over the Kremlin’s full-scale invasion of Ukraine. The episodes of severed cables raised worries that the shadow fleet might also be used for sabotage.