The long-simmering conflict between Russia and Ukraine bubbled over Sunday when Russian border guards opened fire on three Ukrainian vessels in the Black Sea. The Russians then captured the 24-member crew.
Russia says the Ukrainians had violated its border while Ukraine says its ships were acting in line with international maritime rules.
The Tass news agency on Friday quoted Russian government-appointed ombudswoman Lyudmila Lubina as saying that the vessels' commanders are being transferred to Moscow for interrogation. The other 21 remain in custody in Crimea.
A Crimea court earlier this week ruled to keep the Ukrainian seamen behind bars for two months pending the investigation.