Forces on the Marshal Shaposhnikov freed the captured tanker
Ten suspected Somali pirates captured by the Russian navy last week may have perished after their release, a defence source in Moscow has told reporters.
Marines seized them during a dramatic operation to free a hijacked Russian oil tanker far from shore, killing an 11th suspect in the gun battle.
They were released in an inflatable boat without navigational equipment.
Within an hour, contact was lost with the boat's radio beacon, the defence source said.
"It seems that they all died," the unnamed source was quoted as saying by Russia's Interfax news agency.
Russia initially said the 10 pirates would be taken to Moscow to face criminal charges over the hijacking, but they were released instead because there were not sufficient legal grounds to detain them, the defence ministry in Moscow said.
The UN Convention on the Law of the Sea, to which Russia is a signatory, gives sovereign nations the right to seize and prosecute pirates.
Western officials were very surprised when the Russian authorities dropped plans to put the pirates on trial in Moscow, the BBC's Richard Galpin reports from Moscow.
Now there is even more surprise the pirates were set adrift in the Indian Ocean to make their own way home, he adds.
The tanker, the Moscow University, was seized on 5 May some 350km (190 nautical miles) off the Yemeni island of Socotra as it sailed for China, carrying crude oil worth $50m (£33m).
Marines from the Russian warship Marshal Shaposhnikov stormed the ship the following day, freeing the 23 Russian crew members who had locked themselves in a safe room after disabling their ship.
Cdr John Harbour, spokesman for the EU naval force in Somalia, Navfor, said the Russian navy had been within its rights to release the suspects.
It was, he told the BBC News website, impossible to judge their situation without knowing the details of the boat - described as an inflatable by Russian sources - and the radio beacon they had been given.
It was quite likely the Russian ship lost radar contact with the boat after an hour, Cdr Harbour said, while the signal from the beacon would depend on the strength of its battery and whether or not it could be detected by satellite.
The Navfor spokesman suggested the loss of navigational equipment would not necessarily be critical if there was an experienced mariner among the 10 men on the boat.
Stressing that nothing could be said for sure without knowledge of the boat, the weather and other factors, he noted that pirates had been known to operate up to 1,200 nautical miles (2,200km) from the Somali coast.