Shares of the security software company Symantec are plunging 30 percent at the opening bell after the company revealed an internal investigation that could delay its annual report.

Details, released in a filing with the Securities and Exchange Commission late Thursday, say only that the investigation was begun in connection with the concerns of a former employee.

The revelation appears likely to wipe more than $5 billion from the company's market value when the market opens. Shares are down $8.68, to $20.60.

The Mountain View, California, company says the matter has also been referred to the Securities and Exchange Commission, and that the outcome of the investigation could impact its financial results and guidance.