A Russian cosmonaut who explored a mysterious hole in a capsule docked to the International Space Station says Russian law enforcement agencies are investigating what caused the opening.

Sergei Prokopyev said Monday that investigators were looking at samples he and crewmate Oleg Kononenko collected during a Dec. 12 spacewalk. Prokopyev and two other astronauts returned to Earth last week after 197-day space station mission.

The hole was spotted on Aug. 30 in the Russian Soyuz spacecraft attached to the station. The crew located and sealed a tiny leak that was creating a slight loss of pressure.

Roscosmos chief Dmitry Rogozin said in September the hole could have been drilled when the capsule was built or in orbit. Rogozin stopped short of blaming crew members, but the statement has caused friction between Roscosmos and NASA.