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 / Updated  / Source: Reuters
By Reuters

When exactly did Yahoo know about the 2014 mega hack that affected 500 million users?

Democratic Senator Mark Warner on Monday asked the Securities and Exchange Commission to investigate whether Yahoo and its senior executives fulfilled obligations to inform investors and the public about a hacking attack affecting 500 million user accounts.

Yahoo president and CEO Marissa Mayer speaks on stage in 2014. Yahoo is investigating a breach that includes 500 million accounts.Julie Jacobson / AP

"Disclosure is the foundation of federal securities laws, and public companies are required to disclose material events that shareholders should know about," Warner said in a letter to SEC Chairwoman Mary Jo White.

Related: Users Start to Dump Yahoo After Mega Data Hack

Yahoo has faced pointed questions about exactly when it knew about the cyber attack announced last week that exposed the email credentials of half a billion accounts, a critical issue for the company as it seeks to prevent the breach from affecting a pending takeover of its core business by Verizon.

Related: Yahoo Hack Likely to Delay - But Not Derail Verizon Deal?

Warner also asked the SEC to probe whether Yahoo has "made complete and accurate representations" about the security of its information technology systems.