Capt. Timothy Patrick says Hartzell wasn't shot, but the cause of death remains unknown.
Military officials are releasing few details, but say Law also tried to hurt himself. The nature of his injuries have not been released.
Officials are not ready to call it a murder, suicide attempt and no charges have been filed. Law is not in jail, but remains hospitalized.
Both men joined the Marine Corps in 2006 and served in the 2nd Marine Logistics Group.
Hartzell was a graduate of Ross High School near Hamilton in southwest Ohio.