Ukrainian state security forces detained a Russian oil tanker in the port of Izmail on Wednesday, seizing documents and questioning the crew before releasing them.

Ukraine said the action was taken due to the Russian ship’s involvement in the November 2018 Kerch Strait confrontation, which ended with Russia seizing a Ukrainian ship and imprisoning its 24 crew members.

According to Ukraine’s SBU security service, the Russian tanker Nika Spirit was involved in blocking the Kerch Strait last year under the name Neyma. The owners of the ship were accused by the SBU of changing its name to “hide its involvement in the illegal acts of aggression that took place on November 25, 2018.”

The Kerch Strait is the narrow passage that divides the Black and Azov seas and separates the Crimean peninsula from Russia. Russia built a bridge across the strait after annexing Crimea in 2014, a move widely condemned by the international community. The Ukrainians feared the bridge would become a navigational obstruction that made it easier for Russia to block the strait and bottle up Ukrainian shipping in the Sea of Azov, which is essentially what happened in November 2018.

The Russians are still holding the Ukrainian sailors taken prisoner after their ship was seized, despite condemnations from the International Tribunal for the Law of the Sea. During his first telephone conversation with Russian President Vladimir Putin two weeks ago, Ukraine’s new president Volodymyr Zelenskiy asked once again for the sailors to be freed.

Thursday’s events would suggest those negotiations are not going terribly well, although one theory of how the incident played out is that allies of Zelenskiy’s predecessor, Petro Poroshenko, authorized the seizure to scuttle Zelenskiy’s effort to recover the Ukrainian sailors held by Russia.

Not long before the seizure was announced, Zelenskiy’s administration said there had been positive movement on the negotiations, while the Russians indicated their interest in further “dialogue” with the new Ukrainian government. Zelenskiy’s office has not commented on the impounding of the Nika Spirit as of Thursday afternoon.

Russia denounced Ukraine’s seizure of the Nika Spirit as “absolutely illegal” and “state-sponsored piracy,” warning of dire consequences if the ship’s dozen or so crew members were “taken hostage.”

“This is very bad news,” said Russian Deputy Foreign Minister Grigory Karasin on Thursday. “Someone in Kiev wants a tough throwback to their provocation on Nov. 25.”

The SBU stated the current crew of the ship were not accused of violating any laws and were therefore released and returned to Russia immediately.