Guitar-maker Gibson was being investigated after raids at its factories and offices in Nashville and Memphis to seize what federal agents said was illegally exported ebony from India.
Prosecutors confirmed the investigation on Wednesday in a filing that asked a judge to delay efforts by the company to reclaim the wood. After the raid in August, the firm's CEO Henry Juszkiewicz denied wrongdoing and complained in media appearances that the federal government had implicated Gibson without filing charges.
The company has won the support of rightwing groups by citing the raids as examples of overzealous regulation that threatens jobs.
Gibson was the subject of a similar raid in 2009 after the seizure of wood allegedly exported illegally from Madagascar.
The US Attorney, Jerry Martin, declined to provide further details of the investigation by the Environmental Crimes Section of the Justice Department. Prosecution documents, however, point out that the federal government can "seek criminal fines and imprisonment for knowing violations of the Lacey Act", which bans the import of illegally harvested wildlife and plants.
Details of the case are being kept secret so that the investigation is not compromised, according to court documents.