Military drone maker AeroVironment Inc. said late Friday that the U.S. Justice Department is investigating its billing practices.

The Monrovia maker of unmanned aircraft systems said in a regulatory filing and a separate statement that the department’s civil fraud section was looking into billing rates for maintenance work, time and materials on government contracts in fiscal 2006.

AeroVironment said that it is voluntarily cooperating with the investigation and did not expect a material impact on its business. The company noted that the Defense Contract Audit Agency in February had checked its billing practices and determined them adequate. The company said it had not been barred from government contracts and had received more than $50 million in orders from the government since February.

The statement was released just before markets closed. In addition, the company late Thursday reported good fiscal fourth quarter earnings but issued what analysts considered a weak outlook for fiscal 2011.

The news reports sent shares down nearly 8 percent, or $1.83, to close at $21.43 on the Nasdaq.