The unidentified high-ranking Defense Ministry official did not elaborate on how the pirates died, deepening a mystery that has prompted speculation the pirates were executed by commandos who had freed a Russian oil tanker seized in waters 500 miles (800 kilometers) east of Somalia's coast.
The official told Russian news agencies 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 on 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 they'd been released because of "imperfections" in international law. The statement was met with skepticism, especially in light of a comment made by the Russian president.
"We'll have to do what our forefathers did when they met the pirates" until the international community comes up with a legal way of prosecuting them, Dmitry Medvedev said on the day the ship was stormed.
The international community has had difficulty formulating an accepted policy for trying suspected pirates.
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."
The multimillion dollar business of pirate attacks has continued to climb despite the presence of about 35 international warships patrolling the waters off Somalia. The impoverished nation is caught up in an Islamic insurgency and has not had a functioning government since 1991.
The spokesman for the European Union's anti-piracy force said Somali pirates have released a cargo vessel they had held for more than a month.
Cmdr. John Harbour said the pirates left the vessel early Tuesday, three days after they had received a ransom. The refrigerated cargo vessel is sailing north, he said.
Harbour said the crew of 23 Sri Lankans, one Filipino and one Syrian is safe and well.
Somali pirates hijacked the Talca on March 23 about 120 miles (190 kilometers) off the coast of Oman.
