Symantec Corp. (SYMC) - Get Report shares plunged in pre-market trading Friday after the cyber security firm said an internal investigation could delay the filing of it annual report and alter its weaker-than-expected sales and earnings forecasts.

Symantec, which makes the Norton Antivirus and LifeLock security services, said concerns raised by a former employee, which it did not provide further detail, prompted the probe and that it had retained independent counsel and notified the U.S. Securities and Exchange Commission. The revelation followed the Mountainview, Calif.-based group's first quarter earnings report, which included a softer-than-anticipated full year revenue forecast of between $4.76 billion and $4.9 billion and earnings of between $1.50 and $1.65 per share. 

"The investigation is in its early stages and the Company cannot predict the duration or outcome of the investigation. The Company's financial results and guidance may be subject to change based on the outcome of the Audit Committee investigation," Symantec said. "It is unlikely that the investigation will be completed in time for the Company to file its annual report on Form 10-K for the fiscal year ended March 30, 2018 in a timely manner."

Symantec shares plummeted 24.3% in pre-market trading Friday, indicating an opening bell price of $22.09 each, a move that would lop more than $4.2 billion from the company's market value and pull the stock to the lowest level since August 2016.

Customers provide LifeLock with their Social Security number, birthdate and other data, and the company scans for credit applications, mortgage loan applications, address changes and other signs of identity theft. Plans range in cost from $9.99 per month for coverage of up to $25,000 for stolen funds and monitoring one bureau, to $29.99 a month for coverage up to $1 million, beefed-up alerts and monitoring of the three big credit bureaus.

Symantec's investors relations vice president, Cynthia Hiponia, told a conference call late Thursday the investigation "does not relate to any security concern or breach with respect to our products or systems" but wouldn't elaborate further and would not respond to questions related to it owing to ongoing probe.