The Investigative Committee said in a statement investigators questioned at least three Russian activists.
The majority of the group is made up of foreigners and the committee is waiting for interpreters, RIA Novosti said.
The suspects were brought in on the Greenpeace ship, the Arctic Sunrise, which was seized by Russian border guards Thursday.
Guards also detained two activists who climbed up an oil rig to protest oil drilling.
The detained suspects face a possible 15 years for piracy, but Investigative Committee spokesman Vladimir Markin told RIA Novosti the charge of piracy is not final and may change.
The authorities are allowed to keep suspects in detention for 48 hours, but once they are officially arrested, can hold them for months.
Greenpeace has denied the accusations and said the protest was peaceful.