Democratic Senator Mark Warner on Monday asked the U.S. 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.
“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.
Yahoo (YHOO) 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 (VZ).
Get Data Sheet, Fortune’s technology newsletter
Warner also asked the SEC to probe whether Yahoo has “made complete and accurate representations” about the security of its information technology systems.