Shares of Mattel Inc. plunged 9.46% in after-hours trading on Aug. 8 to $12.16 after the U.S. toymaker withdrew its planned bond offering following the receipt of "an anonymous whistleblower letter."

In an SEC filing, Mattel said it was made aware of the whistleblower letter on Aug. 6. It did not provide further details but said it will "investigate the matters set forth in the letter."

The company decided to terminate its offering of $250 million worth of its 6.00% senior notes due 2027, which was scheduled to close Aug. 8, while it conducts the investigation. Mattel had earlier said that it plans to use the offering's net proceeds, along with cash on hand, to redeem and retire all of its 4.530% senior notes due 2020.

The company said it plans to refinance its 4.350% senior notes due October 2020 prior to maturity.