The pirates seized by a Russian warship off the coast of Somalia have been released because of “imperfections” in international law, the Defense Ministry said Friday, a claim that sparked skepticism — and even suspicion the pirates might have been killed.

Authorities initially said the pirates would be brought to Russia to face criminal charges for hijacking a Russian oil tanker. But Defense Ministry spokesman Col. Alexei Kuznetsov told The Associated Press on Friday that the pirates had been released.

Kuznetsov declined to elaborate on the purported legal flaws that prompted the release and it was unclear how the seizure of the tanker might be legally different from last year’s alleged hijacking of the Russian-crewed freighter Arctic Sea.

That vessel allegedly was seized by pirates in the Baltic Sea off Sweden and went missing for several days before a Russian warship tracked it down off West Africa. The eight alleged pirates were flown to Moscow to face eventual trial.

The Law of the Seas Convention, to which Russia is a signatory, says the courts of a country that seizes a pirated vessel on the high seas have the right to decide what penalties will be imposed.