Symantec plunged 20 percent in extended trading Thursday after the security software company said its board is investigating concerns raised by an ex-employee and that the matter has been relayed to the SEC.

"The Audit Committee of the Board of Directors has commenced an internal investigation in connection with concerns raised by a former employee," Symantec said in a press release that included its fiscal fourth-quarter results. "The investigation is in its early stages and the Company cannot predict the duration or outcome of the investigation."

Symantec didn't provide any details surrounding the nature of the investigation but did say that the issue would likely keep the company from being able to file its annual report for the year ended March 30, "in a timely manner."

The stock dropped $5.80 to $23.38 after the announcement.

Symantec's results surpassed Wall Street expectations on earnings and revenues, but its forecast for both the full year and current quarter fell short of analysts' estimates.

Shortly after the announcement, Rosen Law Firm, a firm that represents investors, put out a press release saying that it's investigating whether Symantec "may have issued materially misleading business information to the investing public."