Local prosecutors said the seals, in the cargo bay of an aircraft transporting them from Sakhalin Island, could have been affected by high temperatures if the crew set the thermostat too high for the animals, RIA Novosti reported.
However, an expert from the Oceanological Institute said the seals could have suffocated, as the cargo bay was too small for the 20 adult mammals.
"The average weight of an adult seal is over 300 kilograms [660 pounds]. If there were several animals locked in a cage, they could have crushed each other with their bodies," the expert said.
Veterinarians said they had ruled out disease as a cause of the deaths.
The carrier, Vladivostok Avia, refused to comment on the incident.