Exxon Mobil spokeswoman Treacy A. Roberts said Thursday that the Federal Bureau of Investigation told the company that the shots administered were "definitely not the flu vaccine."

FBI officials did not explain how they found out about the potential fraud, Roberts said.

Exxon Mobil offered blood tests and counseling to the up to 1,000 employees who took part in the health fair at the oil company's vast complex of refineries and chemical plants just east of Houston.

It doesn't appear that the fake shots were harmful, but steps were being taken to ensure workers' safety, U.S. Attorney Chuck Rosenberg said in a statement Thursday.

The FBI and the U.S. Food and Drug Administration are investigating what was in the syringes and whether others might have received the fake vaccine, Rosenberg said.

Jeanne Miller, another Exxon Mobil spokeswoman, said a doctor provided the shots in Exxon's first use of an outside contractor for the health fair. In the past, she said, flu shots at Exxon health fairs have been offered by company medical staff.

Miller declined to identify the doctor because of the federal investigation.