A Russian official claimed on Tuesday that 10 pirates seized by Russian special forces aboard an oil tanker last week were quickly freed but then died on their way back to the Somali coast.

The unidentified high-ranking Defense Ministry official did not elaborate on how the pirates died, deepening a mystery that has prompted speculation that the pirates were executed by commandos who freed the Russian oil tanker seized in waters 800 kilometers east of Somalia's coast.

The official told Russian news agencies that the pirates' boat disappeared from Russian radar about an hour after their release.

"They could not reach the coast and, apparently, have all died," the official said.

The Defense Ministry could not be reached despite repeated phone calls Tuesday to the press office and the cell phones of spokesmen.

Russian officials have said one of the 11 pirates was killed during a gunbattle when the Russian special forces stormed the tanker Thursday. The others, some said to have been wounded, were brought aboard a Russian destroyer.

Officials initially said they would be taken to Russia for trial, but the Defense Ministry said Friday that they'd been released because of "imperfections" in international law. The statement was met with skepticism.

Somalia's ambassador to Russia, Mohammed Handule, told journalists that his government could not identify or locate the pirates. They were believed to be Somalis, but their nationality has never been confirmed.

He said they "will face trial if we find them alive."