Federal officials declined to say whether anything was removed from Gibson's plant or what specifically the agents were trying to find. But some exotic hardwoods traditionally used in making premium guitars, such as rosewood from the rain forests of Madagascar and Brazil, have been banned from commercial trade because of environmental concerns under a recently revised federal law.

Under the U.S. Lacey Act, trading in such banned woods is a federal offense, punishable by civil and criminal penalties or the seizure of property.

Environmental activists say the search of Gibson's plant took them by surprise because the company's top executive, CEO Henry Juszkiewicz, has taken a lead role for at least the past six years in urging the music industry to use only sustainable wood products.

"Historically, Gibson has shown an awful lot of leadership; they are really one of the manufacturers far ahead of the field," said Scott Paul, director of the forest campaign for Greenpeace, the international activist group.

Paul said the investigation at Gibson's facility shows how complex and tricky the sale of wood products can be, especially when some sales are arranged through third parties in remote countries.

"Today proves that even if you're very serious about buying only certified, well-managed supplies, it's still possible to get caught up … in many of these regions where law enforcement is not always great and corruption is not uncommon," Paul said. "There are a lot of middle men between the guitar manufacturer and the company that is logging the ground. There are a lot of people who are not that honest in the timber business worldwide."