Shares of Embraer , the world's third-largest commercial planemaker, suffered the worst drop on Brazil's benchmark stock index on Thursday after reporting weak results and disclosing a bribery investigation.

The stock closed 5 percent lower in Sao Paulo at 11.09 reais a share, its worst session in six weeks, compared with a 1.5 percent rise on the Bovespa index <.BVSP>.

Embraer booked a slim third-quarter loss due to a plunging local currency and warned in a late Wednesday filing it could miss its target for 2011 net revenue by as much as $200 million due to delayed deliveries.

The company also said in the securities filing that it had opened an internal investigation into transactions in three countries in response to a subpoena from the U.S. Securities and Exchange Commission.

The subpoena concerned possible violations of the U.S. Foreign Corrupt Practices Act, which prohibits companies listed in the United States from bribing foreign officials.

Embraer lists American depositary receipts on the New York Stock Exchange.

Reuters reported in September that U.S. law enforcement officials were investigating possible bribery associated with contracts between aerospace companies and state airlines. There was no immediate evidence that the investigations were related.

While the aerospace industry has long been subject to such scrutiny, recent enforcement efforts by the Federal Bureau of Investigation has focused on sales and maintenance contracts on the commercial side rather than in defense.

Recent cases involving defense companies may have spurred investigators to take a closer look at commercial aerospace deals, particularly given efforts by companies to offset declining demand in the United States and Europe with more deals in the Middle East and Asia, where corruption has been rampant. (Reporting by Brad Haynes in Sao Paulo and Tim Hepher in Paris; Editing by Matthew Lewis, Gary Hill)