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.
Surprised? The Russian government acts swiftly and decisively to defend the interests of the Russian people. Only a hyper-lawyered elite who are openly antagonistic to their own nations would be 'surprised.'