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Symantec shares plummeted roughly 20 percent on Thursday after the antivirus maker warned an internal investigation could delay its annual report.

The plunge followed news that the audit committee of Symantec's board had launched an internal investigation into undescribed concerns raised by a former employee. The probe is being aided by independent counsel and other advisers, the company said in its fiscal fourth-quarter earnings report.

Symantec said it had informed the US Securities and Exchange Commission of the probe and intends to share information collected during the investigation with the commission.

The cybersecurity company's stock fell more than 20 percent to $23.04 in after-hours trading after the possible delay was announced. It had bounced back slightly to $23.75 in recent trading.

"The investigation is in its early stages and the company cannot predict the duration or outcome of the investigation," Symantec said in its filing. "The company's financial results and guidance may be subject to change based on the outcome of the Audit Committee investigation."

The company said it is "unlikely" the probe will be finished in time for it to file its annual report for the fiscal year ended March 30, 2018 in a "timely manner."

The company, which makes the Norton Antivirus software, reported revenue of $1.2 billion for the three-month period ended March 30, an increase of 10 percent year-over-year. Its net loss shrank from $143 million in the year ago quarter to $35 million, or 6 cents a share.

Symantec declined to comment beyond information in its filing.

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